GASTRIC GLANDS. 83 



pearance is on the whole unlike that of any known trilobite. In Naraoia, however, the 

 markings are much finer and directly comparable with those of Elyx. If my contention that 

 Naraoia is a trilobite should be sustained, it might almost settle the question of the "ner- 

 vures." In Burgcssia these lateral trunks enter the main canal behind the fifth pair of ap- 

 pendages. In the trilobites they debouch much further forward. 



The principal argument in favor of the interpretation of these markings as nerves lies 

 in their connection with the eyes. There is considerable evidence to indicate that the eye- 

 lines and the genal caeca are two distinct structures, but because both originate from the 

 sides of the anterior lobe of the glabella, and both extend outward at nearly right angles 

 to the axis, or obliquely backward, they are, when both present, coincident. Genal caeca 

 occur on blind trilobites, on trilobites with simple eyes, and on trilobites with compound eyes. 

 Eye-lines occur on trilobites with both simple and compound eyes, and genal caeca may or 

 may not be present in both cases. The morphology of the ridge forming the eye-line in 

 trilobites with compound eyes is well known. It is abundantly proved by ontogeny that it 

 is the continuation of the palpebral lobe, and a development of the pleura of the first dor- 

 sal segment of the cephalon. Lake, Swinnerton, and Reed have tried to show that the eye- 

 lines of the Harpedidae and Trinucleidse are homologous with the eye-lines of the trilobites 

 with compound eyes, and that the ocelli on the cheeks are therefore degenerate compound 

 eyes. 



The simplest form of the genal caecum is seen in the blind Elyx (Lindstroem 1901, pi. 

 6, fig. 43). The main trunk is at nearly right angles to the axis, the increase in its width 

 is gradual in approaching the glabella, and an equal number of branches diverge from both 

 sides. 



Ptychoparia striata (Barrande 1852, pi. 14, figs. I, 3) is an excellent example of a trilo- 

 bite with compound eyes and genal caeca. It will be noted that the main trunk and the eye- 

 line are coincident, and that both on the free and fixed cheeks the branches are all on 

 the anterior side of the eye-line. Compare this with the condition in Conocoryphe 

 (Barrande, pi. 14, fig. 8; Lindstroem, pi. 6, fig. 44), and one sees there a main branch 

 having the same direction as in Ptychoparia and likewise with all the branches on the anterior 

 side. At first sight this would seem to support the contention that these lines do lead out 

 to the eyes, since Conocoryphe is blind, and the main trunk leads practically to the margin. 

 But although Conocoryphe is blind, it has free cheeks, and the main trunk does not lead to 

 the point on those free cheeks where eyes are to be expected, but back into the genal angles. 

 And this direction holds in such diverse genera (as to eyes and free cheeks) as Harpes, Crypto- 

 lithus, Dionide, and Endymionia. In all these the genal caeca fade out in the genal angles, and 

 in none of them would compound eyes be expected in that region. The coincidence of the 

 eye-lines with the trunks of the genal caeca in Ptychoparia seems to be merely a coincidence. 

 That the markings which radiate from the eyes of Ptychoparia and Solcnopleura are not im- 

 pressions made by nerves is obvious. That they are of the same nature as the similar mark- 

 ings in the eyeless trilobites is equally obvious. Ergo, they can not be nerves in either case, 

 and that they have anything to do with the eyes is highly improbable. The eye was merely 

 superimposed upon these structures. 



The relation of the genal caeca to the ocelli on the cheeks is best shown in the Trinu- 

 cleidae. In all species of Tret as pis simple eyes are present, and in most of them there are 

 very narrow eye-lines. The latter are occasionally continued beyond the ocular tubercle back 

 to the genal angle. A similar course is seen in Harpes. If the simple eye is the homologue 



