Scutum

“ So that in both scutum and tergum there is an articular furrow, bordered in each case, on one side by the margin of the valve, and on the other side by the so-called articular ridge. In Chelonobia (Pl. 14, fig. 1 b) the articular ridge of the scutum is horny. When, as often happens, the scuta and terga have been much worn, the manner of their articulation (Pl. 18, fig. 1 a) is pretty well shown even from the outside; in this case their external appearance is very different from what it is in those individuals (fig. 1 c) of the same species, which have not suffered disintegration. ”

“ When surface markings exist on the scuta, they are very useful in defining species. The color is also a good character in some groups. Where one of these fails us the other very generally assists. The form of the last scutum is sometimes distinctive. The best characters, however, are those derived from the genital organs. The form, &c., of the antennæ and head often afford some clue.
Genus POLYDESMUS, Latreille. [24]
Meatus laterales in scutis 5—7—9—10—12—13—15—16—17—18—19—positus. ”

“ That portion of the scutum which corresponds with the valve in ordinary cases, and alone is externally visible as long as the operculum is united by the opercular membrane to the sheath, is narrow, with the basal margin considerably hollowed out: the occludent edge is formed into thick teeth. The Tergum is elongated, rather exceeding in length the scutum, the latter being measured from the apex to the rostral projection of the adductor plate. The surface of the valve is depressed in the line of the spur, with the basal end of the latter bluntly pointed. ”

“ SCUTUM AMBIGUUM (Plate IX.) .—Fig. 23 (late Parmophorus unguis) is a white shell, covered with a thin brown epidermis, and is sometimes over 21⁄2 inches long. The animal is like a big black slug, and, in comparison with the size of the slug, the shell is very small. A slug the size of a man’s fist would have a shell about an inch long. Most shell-hunters would pass by a Scutum abiguum, not thinking it had a shell embedded in its folds. The shell is found amongst rocks in sheltered places on ocean beaches. ”

“ Scuta oval, of varying proportions; the basal margin is generally narrow, and blends into the [Pg 101] carina-tergal margin; the internal basal rim generally is well developed, sometimes with, and sometimes without internal teeth beneath the umbones. In P. eburnea, and sometimes in P. crassa, there is a line of apparent fissure, and in P. fissa of actual disseverment, running from the umbo to the apex of each scutum, nearly in the line in which a ridge extends in Lepas: the primordial valves of the scuta in these three species, are seated at the basal angles of the lateral and larger segments. ”

“ The basal portion of the mandibles on each side project, so that if a vertical section of the head through the middle were taken it would be an oval with its greater diameter transverse. The eyes are in strictly triangular patches, with remarkably straight sides. The scuta are so deeply canaliculate along their dorsal centre as to be almost divided into two parts. The dorsum is not perfectly rounded, but there is somewhat of a ridge or angle in the centre. Each scutum is furnished with a regular series of distant large puncta, or perhaps more properly, pores. ”

“ It would appear that the present species does not attain quite so large a size as the more northern V. Strömia; the walls are not longitudinally ribbed as is usual with this latter species. The proportional sizes of the compartments seem to be somewhat different; the fixed scutum is either equal to or even smaller than the fixed tergum, instead of being larger, as in V. Strömia; but in young individuals the proportions are reversed. In several specimens the fixed scutum and tergum together were larger than the carina. The rounded adductor plate of the fixed scutum is extremely large. ”

“ This structure, as well as that of the crenated scales on the peduncle, is important, for by this means the animal, as we shall presently see, forms and enlarges the cavity in the rock or shell in which it is imbedded.
The scutum overlaps either about one third or even one half of the entire width of the tergum, and abuts against a prominent longitudinal ridge on its exterior surface. In L. truncata and L. Valentiana, this ridge on the tergum being folded over towards the scutum, forms a conspicuous furrow, receiving the tergal margin of the latter. ”

“ Extraordinarily great as is the difference between the right and left sides of the whole shell, yet in all the species it seems to be entirely a matter of chance whether it be the right scutum and tergum with the right side of the rostrum and carina, or the left scutum and tergum with the left side of the rostrum and carina, which become abnormally developed. ”

“ The outer edges of each scutum are prolonged into curved hooked spines. The first scutum is the shortest and broadest, and has the heavier spine or hook, while the last two segments often lack the hook. The abdomen is finely granulate and at the lower edge of each scutum there are eight heavy, short, simple hairs.
Pedipalps: Longer than body, coxa smooth, trochanter with large protuberance ending in a strong spine on outer side; femur longer than cephalothorax, slightly concave on inner edge, convex on outer edge. Tibia pedicellate, shorter than femur. ”

“ Scutum: the lines of growth are crenated, causing the surface to be very obscurely striated longitudinally: the articular ridge is very prominent, as can be best seen from the outside, and runs down the whole length of the tergal margin with a very regular curve, and hence differs from the articular ridge in the foregoing species. ”

“ It is noticeable that the terminal scutum has a single median, instead of two lateral sutures. The preanal scale is rather large, much narrowed, and very slightly emarginate posteriorly. Where the marginal spines of the lateral anal appendages exceed two in number, some of them are generally very small and situated at or near the base of the terminal process. I have named this species in honor of Prof. Edward D. Cope, with whom it has been my good fortune to have been associated since the earliest dawn of our tastes for natural history. ”

“ The præscutum and postscutellum are usually minute and crowded down out of sight between the opposing segments. As seen in Fig. 90, the præscutum of most moths (Telea) is a small rounded piece, bent vertically down so as not to be seen from above. In Polystœchotes and also in Hepialus the præscutum is large, well-developed, triangular, and wedged in between the two halves of the scutum. The postscutellum is still smaller, usually forming a transverse ridge, and is rarely used in taxonomy. ”

“ The last scutum is, however, without rugæ, and but slightly roughened. It is much narrowed anteriorly and posteriorly, and somewhat elongate. The preanal sternum is almost circular, with two lateral curved impressions and a central shorter straight one, which are probably the remains of the sutures between the plates of which it was composed during embryonic life. The articular spines are rather strong. The smallest specimen has only twelve pairs of eyes. It affords me great pleasure to dedicate this species to Mr. ”