Gustaf Lindström

“ When the free cheek is dissevered it shows no trace of the ridge, there is only a large scallop on the spot where it embraced the ridge. In the Paradoxidæ the rim of the indenture and the ridge are in so close contact that there is not the least place for an eye between them, as can be seen in the few specimens with a complete head. In all oculate trilobites again without any exception the facial suture separates that part of the eye which is the real visual organ with corneal facets, from the interior often elevated portion, opposite it, the so called palpebral lobe. ”

“ On its surface there are two small, resplendent, white or clear spots, almost of the shape of a crescent, though more irregular when seen in higher enlargement (pl. VI fig. 49) . They are translucent as to be visible on the interior side of the hypostoma. In Apus glacialis there are also similar spots in the same position, but they form short narrow, straight stripes, parallel with the border of the posterior margin. I have no opinion as to the nature or function of these white spots. I do not think that they at all are homologous with the macula; of the trilobites. ”

“ Consequently the curious appendages on the inferior side of the lenses in Dalmanites vulgaris (Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways be considered as belonging to the original structure of the eye, apart from their great dissimilarity with anything appertaining to the eyes of the Arthropoda. The cornea on the contrary cohered with the integument of the body, and it has been well preserved in a great number of trilobites. ”

“ I cannot but think that this is a destruction which has set in long after the fossilization. It has revealed certain states of the intimate structure, certain delicate details, that now with an astonishing regularity come in sight and probably also lie hidden in the intact prisms. In the specimens of Asaphus, which we have studied, the alteration has taken the shape of a concentric stratification forming the body of the prisms, which is well discernible in a horizontal section, but not easy to detect in the longitudinal one (Pl. I figs 9-10, 11) . ”

“ They do not continue deep down.
If we now compare the reticulate zone of the eye with the maculæ of the hypostoma, for instance the vertical section fig. 16 with fig. 11 b, we find the most complete identity in structure. In the same manner the horizontal section of the border zone of the eye, fig. 8 a, fig. 10 b, and of the macula, fig. 17 are similar. The same chaotic, spongious mass in both, with some tendency to form prisms more evident in the longitudinal sections, where the same reticulate structure with predominant white, longitudinal streaks is so palpable. ”

“ The facial ridge continues in a great variety of shapes, short or long, but always forming the fraction of a circle, always of nearly equal thickness, only slightly tapering towards one of the extremities, and always when in direct connection with the glabella, starting from the base of its foremost, largest segment. ”

“ In the rule the passage from the prismatic surface to the reticulate is gradual, the prisms becoming by and by irregular in their outline and diminishing in size (fig. 12, a the eye proper, b the border zone) , but there are also instances where the distinction between these two fields is sharp and without any gradual transition. In the inferior reticulate zone there are generally some oblongue, funnelshaped pits. I am uncertain whether they are to be regarded as regular parts of the eye structure or rather as burrows of some parasite. ”

“ The free cheek around the eyes does not form a border zone, somewhat imitating the eye structure as in Asaphus, but is more compact, composed of vertical elements which give to the test of the trilobites in general a tendency to split up in vertical prisms.
Of a peculiar interest are the eyes in the oldest of all oculate trilobites, at present with certainty known, Eurycare, Peltura, Sphærophthalmus and Ctenopyge.
Of these genera Eurycare is the oldest (see table p. 22) . Amongst the many free and detached cheeks only a single, very little one has been found with the eye ball fixed. ”

“ The precarious state of preservation prevents all definite conclusions in that respect. It seems, however, that the evidence gathered through the examination of numerous specimens rather points in a negative direction. As the free cheeks in these old Cambrian trilobites have been in a very loose connection with the fixed cheeks and generally deciduous, contrary to the condition in the Silurian ones, it is in many instances very difficult to tell whether species with facial ridge, especially those from the earlier Olenus schists have been blind or provided with eyes. ”

“ As a rule the ridge is more developed in the young or larval individuals, continuing from the glabella to near the -17- posterior cephalic border in an uninterrupted arch [13] quite as in several of the adult Olenellidæ of the oldest Cambrian. It can be taken as granted that its origin is the same as in the Olenellidæ though at present the only evidence at hand is the small larva of Paradoxides oelandicus, which Linnarsson called Parad. ”

“ In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma and Ceratopyge occur as the last survivors of the blind, partially ridge bearing genera. The Trinucleidæ and Ampyx belong to another group of trilobites.
Even among other exclusively Lower Silurian genera, in which the plurality of the species is oculate, there are species entirely blind. So with Illænus, in which genus Dr Holm has not found any eye in Ill. Angelini, I. leptopleura and Ill. cæcus. The free cheek in these three species is much narrow, as the facial suture lies near the margin of the head. ”

“ In so far as the cornea and its facets or lenses are to be regarded, there is the greatest analogy with the Isopoda. In vertical sections of Sphæroma we have the same sort of elongated, flattened biconvex lenses as in Sphærophthalmus and others. -33- Since Grenacher and Exner and others have published their excellent works on the eyes of the Arthropoda, there can be no foundation for speaking of the resemblance of the trilobitic eye with that of Limulus, as this genus stands completely isolated amongst all Arthropoda in that respect. ”

“ The Olenellidæ belong chiefly to the oldest of the Cambrian beds with trilobites, and none of them has as yet been found higher up.
Next in order of evolution we have the important tribe of the Paradoxidæ. These are preeminently distinguished from the Olenellidæ through the well developed facial suture, which without exception in them all runs outside the facial ridge and separates this from the free cheek. ”

“ The lenses of the cephalic eye are nearly square prisms sometimes with a slight approach to hexaedral pillars. There is no clear transition into a spongious or reticulate border zone. But this may depend upon the bad preservation.
Asaphus fallax Dalm.?
Pl. I fig. 18-22.
It is very difficult to distinguish this species or rather variety from A. expansus, but if we have found genuine specimens, there are some points in the shape of eyes and maculæ which make it different. The hypostoma (f. 21) is rather more broad and the duplicature is large with an upturned margin. ”

“ It now became of importance to ascertain whether the intimate structure also corresponded in both, and microscopic sections were accordingly prepared. Although not so clear as in other species of this genus a vertical section of a cephalic eye (Pl. II fig. 21) shows a median row of black spots which compared with other sections (Pl. II fig. 9) must be the centres of the badly preserved lenses. This is confirmed by the horizontal section (Pl. II fig. 20) where the well known image of the polyedric facets appeared. ”

“ The structure of the cephalic eyes (Pl. 1 f. 1-2) is prismatic, but the separate prisms are rather short and broad. Their lower or interior end is convex. The separating lines between the single prisms are not always distinct.
-37-
Agnostus Dalm.
Pl. 1 fig. 7.
Agnostus glandiformis Ang. Although there is not the slightest evidence of eyes in this the largest of its genus, nor any free hypostoma hitherto has been found, we may here give a little account of our researches into this species. The scantiness of material forms a chief obstacle to our knowledge. ”

“ The enormous hypostoma of this giant resembles in a high degree that of the Asaphi, but is at the same time the most evident verification of the experience that the hypostoma per se cannot be regarded as the sole criterion for determinating the generic affinities of different species. The whole structure of this trilobite in other respects gives it a quite independent position, distinct, from Asaphus. ”

“ In the numerous specimens of which we have examined the hypostoma, there are always small, nearly circular maculæ which by their lighter colour are marked out from the surrounding smooth space of the inferior sinus of the lateral grooves where they are situated. They do not rise at all above the surrounding surface from which they are separated only by a fine, scarcely perceptible line. ”

“ It needs scarcely be mentioned that the genera now enumerated have hardly anything in common, beside the general character of the head, and that real affinity exists only between Agnostus and Microdiscus, and probably also between Conocoryphe, Ctenocephalus and Elyx.
Beecher [2] asserts that there is a suture in Agnostus, but in vain we have searched for it in numerous well preserved specimens and Dr Holm also denies its presence. -11- Nor are there any signs of closed up sutures, which also could not possibly be expected in so early a stage of evolution. ”

“ It is, however, not known how the pleura in the olenellid glabella have been developed, whereas it is well ascertained by the growth of the glabella in Sao, Liostracus and others that such pleura never have been developed in them.
What the phylogenetic evolution has taught us concerning the formation of the facial ridge is confirmed by the ontogenetic development of individuals of some species as for instance Sao hirsuta Barr., Liostracus sp. Brög. ”

“ Sphærophthalmus and Ctenopyge with enormous hemispherical eye balls, while Olenus and Parabolina, their earliest predecessors, probably were -23- blind. We have not amongst hundreds of specimens of these genera found a single specimen showing an ocular globe covered with facets. Olenus and the nearly related genus Parabolina are found in innumerable specimens in the thinly laminated alumschists of Scania and other provinces of Sweden. But rarely a perfect head shield, or nearly so, is to be found with the free cheeks in place. ”

“ But, as Barrande has shown, [38] there is a certain species of Trinucleus, the larva of which wants a facial ridge and eyes, as there also are several adult forms without ocelli. These have remained on a much ancestral stage, while the larva with eyes are more highly developed in such species, where the adult have been subject to a retrograde development. ”

“ It is likewise so in Nileus, where Nileus armadillo has an exceedingly thick exterior integument above the lenses. Such an integument has in a still higher degree increased in Bumastus sulcatus so as to exceed in thickness the stratum of the corneal prisms and it may in fact be doubted if the eyes of this species ever were able to function as visual organs. Proetus nearly resembles Bumastus in the thickness of the integument covering the prismatic lenses, which are interiorly convex, with a diameter of 0,03 mm. ”

“ We have not succeeded in finding any well preserved hypostoma.
The lenses of the cephalic eyes resemble much those of the preceding species though more elongate and ovate. They are covered by a membrane which surrounds their upper moiety and between the lenses joins the interstitial test. In the horizontal section these membranes surround the lenses as an annular wall. ”