MUSEUM OF COMPARATIVE ZOOLOGY 221 



is much larger here than in Phrynosoraa, and the vesicle can scarcely 

 be said to be embedded in the connective tissue of the brain roof, 

 as in the case of Phrynosoma, but rather is suspended from the under 

 side of the wall in a connective-tissue capsule. 



The most striking features about this vesicle, as seen in the section, 

 are its dorso-ventral flattening, and the entire separation of the lens 

 from the retina. The lens, a well defined structure, composed of much 

 elongated, almost fibrous, non-stainable cells, has its margins widely 

 separated from the retina, and the intervening space is occupied by a 

 uniformly fine granular substance (coff.T), which also occupies the nar- 

 row space corresponding to what would be the cavity of the ve'sicle, 

 were the lens and retina continuous at the margins of tlie former. The 

 retina shows no structure beyond two deeply pigmented layers, cor- 

 responding to its external and internal surfaces, connected at short but 

 irregular intervals by pillars of pigment, between which are seen a few 

 scattered nuclei. This distinct separation of the margins of the lens 

 from the retina is the only undoubted case of the kind, so far as I know, 

 that has been seen, and if normal may be of significance in connection 

 with the theory of the origin of the eye recently advanced by Beard 

 ('89). I am, however, inclined to believe, notwithstanding the fact 

 that the condition here found is apparently confirmed by the sections 

 of my second specimen of this species, that the separation is in reality 

 due to the extreme differentiation of the two structures, by means of 

 which the connection between them was weakened, and then to artifi- 

 cial rupture by the flattening of the vesicle. The point certainly needs 

 confirmation in more carefully preserved specimens. 



I was unable to study the epiphysis in the material which I had, but 

 no trace of anything like a nerve or even like a connective-tissue string 

 extending from the parietal vesicle could be detected, nor were there 

 any indications of blood-vessels or sinuses corresponding with those 

 existing in Phrynosoma found here. 



Conclusions. 



The general bearing of the facts here presented I discuss at present 

 only in connection with the question of the function, past and pres- 

 ent, of the parietal organ. I concur in the opinion held by most of 

 the persons who have written on the subject, that the organ is a de- 

 generate eye, although my observations furnish, perhaps, no evidence 

 in addition to what has been presented by former writers, in support 



