62 BULLETIN OF THE 



as representiug the " choroid gland." The evidence for this is princi- 

 pally in the pt)sition which it occupies, and very little in its structure. 

 It is true that this body may contain a small amount of pigment in the 

 normal eye, as I find to be the case in Clevelandia, yet its characteristic 

 structure consists, as is well known, in its richness in blood-vessels ; but, 

 as already said, none of these occur in Typhlogobius. Its position — viz. 

 at the proximal pole of the eye at or very near the entrance of the optic 

 nerve, and between the chorio-capillaris and the sclerotic — is, however, 

 strong evidence in favor of regarding it as the " choroid gland." This 

 structure is described in text-books (Wiedersheim, '86, p. 412) as being 

 situated in the normal fish eye between the argentea and the pigmented 

 layer of the choroid. The fact that no argentea is present in the eye of 

 Typhlogobius weakens somewhat the force of the evidence that I have 

 used to support the assumption that I have made with reference to the 

 significance of the pigment mass described. But its relation to the 

 chorio-capillaris and the pigment layer of the choroid are the same as 

 that of the " choroid gland " ; as is also its relation to the sclera, with 

 the exception that no argentea is interposed between the two. Of 

 'course it is impossible to say that, were the argentea developed, it would 

 lie between the sclera and the pigment mass, rather than between the 

 latter and the pigmented layer of the choroid. We however have as 

 much reason to suppose it would occupy the former position as the latter. 



The pigment layer of the retina is exceedingly thick. In a specimen 

 about 50 mm. long, the entire thickness of the retina including the pig- 

 ment layer was 0.108 mm., and that of the pigment layer was 0.07 mm.; 

 while in a specimen of Clevelandia of about the same length, the entire 

 thickness of the retina mcluding the pigment layer being 0.13 mm., 

 the thickness of the pigment layer was only 0.037 mm. ; that is, in 

 Typhlogobius the thickness of the whole retina is to the thickness of the 

 pigment layer as 1.5 : 1, while in Clevelandia the corresponding ratio is 

 3.5 : 1. In Gasterosteus, I find that about the same pi'oportion holds 

 as in Clevelandia, whereas in the perch (H. Milller, '57) the proportion 

 is at least not less, and in the salmon a year old (Hoff'mann, '83) the 

 proportion is 6-f : 1. 



The layer is composed wholly of pigment of the dark brown granular 

 variety. I have searched in vain for cellular elements within it. In 

 most specimens the pigment is a very uniform mass ; but occasionally 

 one finds an eye in which very distinct and perfectly round nodules of 

 pigment occur. Some of these are so clear-cut and smooth in outline 

 that they have the appearance of perfectly round cells wholly trans- 



