Coracoid

“ There are present, distinctly preserved in the type specimen, the scapulæ, the clavicles, and the interclavicle, with the possibility of the coracoid. The peculiar element referred to by Cope as resembling a "lacertilian pubis" is without doubt the left scapula of the animal (plate 16, fig. 1) . Its form compares very favorably with that of Ceraterpeton as figured by Woodward (630) . The coracoid may be represented by the fragment which lies close to the scapula. The sculptured element lying next to the supratemporal horn of the skull is the right clavicle preserved bottom side up. ”

“ Post-temporal forked; scapula and coracoid more or less developed, sometimes much reduced, the former pierced by a foramen; pectoral rays attached to 4 or 5 hour-glass-shaped pterygials, one or two of which are in contact with the coracoid. Ventral fins jugular, with not more than 4 rays, or absent. Body more or less elongate, sometimes Eel-shaped, naked or with small scales. Dorsal and anal fins elongate, the former constituted entirely of spines, or anteriorly of spines or non-articulated rays, and posteriorly of soft rays. Caudal fin usually distinct, with expanded hypural. ”

“ In the German genera it closely resembles specimens from the Cambridge Greensand.
23 specimens are exhibited. Nos. 4, 10, 12, are the middle parts of shafts of left coracoids. Nos. 3-12, 22, are the middle parts of shafts of right coracoids. Nos. 2, 5, 14, are proximal ends of left coracoids. Nos. 1, 6, 8, 9, 23, are proximal ends of right coracoids. Nos. 15, 16, 17, 18, 19, 20, 21, are distal ends of left coracoids. No. 13 is a nearly perfect left coracoid, and No. 7 is the glenoid cavity for the humerus formed by a right coracoid with the anchylosed scapula.
SCAPULA.
Pl. ”

“ Location.—The coracoid is located just back of the clavicle and at the side of the entrance of the thoracic cavity. It is the strongest bone of the shoulder girdle, extending upward, outward, and forward. It articulates inferiorly with the sternum and superiorly with the humerus and the scapula, and is attached to the superior end of the furcular limb by a fibrous cartilage. ”

“ Left scapula in lateral (G) and medial (H) views. Dashed lines represent suggested missing areas. The scapular blade would have been oriented at approximately 45° to the coracoid. The cranio-dorsal margin of the coracoid was most-likely very thin as the corresponding region of the cranial fossa is only 1–2 cm thick. The distal end of the scapular blade is expanded to a similar degree to that of the proximal end. Caudal of the scapular glenoid expansion, the scapular blade expands into a small tuberosity, a ventro-medial process below the scapular ridge. ”

“ Moreover the angle formed by the junction of the glenoid and scapular articulations (fig. 2) approaches that of T. vicina. I presume, however, that the great expansions of the extremities of fig. 2 (to wit, the beetling roof of the glenoid cavity and massive proportions) make the coracoid in question one of the largest as compared with recent land-tortoises.
A portion of a left scapula, from Zebbug, of a tortoise a good deal smaller than the owner of the coracoid just described is represented in Plate VI. figs. ”

“ The coracoid is one of the most characteristic bones of the bird’s skeleton. Its upper end forms the acrocoracoid process, against the inner surface of which leans the proximal portion of the clavicle. ”

“ From near its ventral end the scapula sends off a long process, which converges transversely with its fellow. This process, the clavicle (!) or the precoracoid of many authors, is the acromial process, the Plesiosauri giving the clue as to how an acromion can assume such an abnormal position. The coracoid, with a suture between it and the scapula, is very long and extends horizontally backwards, not meeting that of the other side. ”

“ Lying anterior to the interclavicle and overlapping its antero-lateral margins lie the two clavicles, which are usually diamond-shaped and are sculptured, dermal bones. The position of the coracoid is still uncertain, and in fact its clear association in the pectoral girdle of these species is still a question. It seems to be constant in the European (186, 251) species and is usually represented by a small rounded plate of bone, which in life no doubt had a large amount of cartilage to form its borders. ”

“ A third bone, or possibly two, on each side, are represented in most mammals by mere vestiges, which early unite with the scapula to form the coracoid process. In the lowest living mammals, of which Ornithorhynchus and Echidna are the only examples, there are in addition to the clavicles three well-developed bones on each side, the scapula and two bones articulating with it at its lower end, the anterior of which, originally named epicoracoid by Cuvier, is generally known as the procoracoid ”

“ As exemplified by Chorismochismus dentex the coracoid is well developed, [512] and, as usual, composed of two pieces, the upper of which is not suspended from the humerus, but fixed by a ligament to the hinder margin of the carpal bones. It is a broad lamella, dilated posteriorly into the cartilage, which is externally visible; the lower piece is narrower, and fixed to the extremity of the pubic bone of its side. The pubic bones are united by suture, and form together a heart-shaped disk, the point of which is produced backwards. ”

“ The scapular arch of the Sharks is formed by a single coracoid cartilage bent from the dorsal region downwards and forwards. In some genera (Scyllium, Squatina) a small separate scapular cartilage is attached to the dorsal extremities of the coracoid; but in none of the Elasmobranchs is the [70] scapular arch suspended from the skull or vertebral column; it is merely sunk, and fixed in the substance of the muscles. ”

“ In Primates the clavicle and coracoid process are always well developed. In Man and the Gorilla the scapula has a long straight suprascapular border, a well-developed coracoid process and spine, and a large curved acromion. Vestiges of the precoracoid occur at each end of the clavicle. The shape of the scapula varies much in the lower Primates.
The Upper arm and Fore-arm.
In the Monotremata the humerus is short, very broad at each end and contracted in the middle. The radius and ulna are stout and of nearly equal size, while the ulna has a greatly expanded olecranon. ”

“ The coracoid bone is notched above the pectoral fin, the notch being terminated below by a spine, and above by an acute corner. There are no scales between the cranial ridges on the top of the head, nor in the concave inter-orbital space. A single row of five or six scales traverses the cheek below the infra-orbitar ridge. The temples before the upper limb of the preoperculum are densely scaly, as is also the gill flap above the upper opercular ridge. ”

“ Examining for this purpose the humeral extremity of the bone, and collecting all the examples, I found it was composed of two elements ossified together, these elements concurring on one edge to form an articular cavity. Of these elements the broader and shorter one, which extended toward the sternum, was coracoidian in form, and perforated in each of four specimens. If, as appeared now to be the case, this was the coracoid, surely the great heavy bone so long called by that name was a pelvic bone, and the restoration of the skeleton must proceed on an entirely new basis. ”

“ The posterior of the two coracoids also disappeared in late Triassic times, in reptiles at least, though a vestige may possibly be present in our own shoulder girdle. The scapula, clavicles, and anterior one of the two coracoids, the so-called procoracoid, are still present in most reptiles; in snakes only are they wholly absent, though much reduced and non-functional in some lizards. ”

“ The ribs are all intercentral in position; the anterior ones very broad near the base, recalling the broadly expanded ribs described by Schwarz (540) for Scincosaurus, Ptyonius, Thyrsidium, and other genera. Posteriorly the ribs become slender and cylindrical. They are all rather long and distinctly curved, with probably a cartilaginous tip.
There is preserved a single element of the right side of the pectoral girdle. This is, I think, the coracoid, an element which has hitherto escaped observation among the American Microsauria. ”

“ These two separate bones have been met with in the embryo of Lepus, Sciurus, and the young of various other mammals belonging to very diverse orders, such as Edentates and Primates. The separation even occasionally persists in the adult. The question is, What is the relation of these bonelets to the coracoid of the Monotremata and to the corresponding regions of reptiles? ”

“ As in other American Edentates, the acromion joins the coracoid. This connexion occurs in both the Two-toed and the Three-toed species. The limbs of these creatures are very long, a concomitant of an arboreal life. The femur has no third trochanter. The genus Bradypus, which by reason of the fact that it has not lost the third toe on the manus seems to be more primitive than Choloepus, shows another structural feature which does not bear out this conclusion. The trapezoid and the os magnum of the carpus are united, while in Choloepus they are perfectly distinct bones. ”

“ The ulna has a hook-like head, the olecranon (o.) which distinguishes it easily from the distally thickened radius. The limb is attached to the body through the intermediation of the shoulder-blade (scapula, sc.) a flattened bone with a median external ridge with a hook-like termination, the acromion (acr.) . There is also a process overhanging the glenoid cavity (g.) wherein the humerus articulates, which process is called coracoid (co.) ; it is ossified from two separate centres, and represents a very considerable bone in the bird, reptile, and frog. ”

“ The points in which it is distinct from birds are that the bone is not produced proximally beyond the glenoid cavity for the humerus, which, instead of being lateral as in birds, and looking outward, in Pterodactyles forms the proximal-termination of the bone. The sternal articulation is proportionally much shorter transversely in Pterodactyles, terminating in a convex margin which rounds up into the thin outer margin, as in the immature coracoid of the common Cock. It is bow-shaped in front instead of being straight, and is commonly longer than in birds. ”

“ It sometimes fails to appear at all.
The centres for the scapula are shown in the accompanying figures (fig. 19) . G. B. Howes regarded the subcoracoid centre as the atavistic epiphysis representing the coracoid bone of lower verte-
1 By mesenchyme is meant that part of the mesoderm, or middle layer of the embryo, in which the cells are irregularly scattered in a matrix, and are not arranged in definite rows or sheets as in the coelomic membrane. ”

“ Zool. Soc. Lond. Part III. 1835, in which he points out many approximations in the generative and other systems of this animal to the organization of reptiles. ↑ In both these animals there is superadded to the ordinary type of bones in quadrupeds, an enlargement of the coracoid bone (c,) and a peculiar form of sternum, resembling the furcula of birds. In Pl. 12, Fig. 1, a. represents the peculiar sternum or furcula; b. b. the clavicles; c. c. the coracoid bones; d. d. the scapula; e. e. the humeri; f. g. ”

“ It is true that such an element as the epicoracoid has not been found ossified in the early reptiles, but neither have numerous other bones in the mesenchyme of mammals, and its ossification in mammals would be nothing remarkable. A comparison of the epicoracoid of lizards (Fig. 99 b) with that of monotremes will show their identity in relations. And doubtless a similar epicoracoid filled in the interval between the coracoids above the clavicles and interclavicles in the early reptiles (Fig. 96 d) . ”

“ At each belt the threads of corium within the parietal pores lend off minute branches to supply the thickened wall. These belts, which continuously surround the shell, correspond (as is best seen in young specimens) , with the little knobs or beads, which, in Coronula (Pl. 16, fig. 4) , rise separately, and not quite regularly, on the longitudinal parietal septa, and which, I believe, are formed at every successive period of growth; here they are much larger, stand in straight transverse rows, become confluent, and are formed only at occasional intervals. ”

“ When granules are present in the flattened cells of the corium, they occupy the nuclear region. They lie in short thin lines while those of the under portion of the Malpighian layer occur in oval groups. Where these granules occur in the rete layer, they cluster around the nuclei. In the colored skin there are dark pigment granules found in the corium and to a less extent in the rete layer. Hanau has described a definite cellular body which he found densely packed among the black-brown granules. There is a central body which sends out branches in all directions. ”

“ The corium is coarse in the centre of the skin where the fibres, which are of the kind known as white connective tissue, and which exist in bundles bound together with yellow elastic fibres, are loosely woven, but towards the flesh side they become more compact, and as the hyaline layer is neared the bundles of fibres get finer and finer, and are much more tightly interwoven, until finally, next the grain itself, the fibres no longer exist in bundles, but as individual fibrils lying parallel with the grain. ”