EYE OF NOTORYCTES TYPHLOPS. 559 



insomuch that, as a rule, no remnant of iris epithelium, pupil, 

 lens, vitreous humour, or hyaloid membrane can be seen, the 

 unbroken wall of pig'meut forming the front of the e3'e. In 

 this condition the eye of Notoryctes is almost exactly com- 

 parable to that of Troglichthys, which Eigenmann claims as 

 the most degenerate of all vertebrate eyes. In the less 

 degenerate specimens, however, structures are present which 

 appear to be the remnants of those parts. Thus in one eye, a 

 little to the outer side of what may be taken as the optic axis, 

 is a slight gap, in which the pigment epithelium is much less 

 dense than around it. This gap is tubular, sloping obliquely 

 outwards and forwards ; in it posteriorly can be seen a double 

 layer of cells Avith oval and flattened nuclei, lying, not edge 

 to edge, but obliquely against one another. These are 

 continuous internally on either side, with a similar layer 

 sometimes two or three cells thick instead of one, and 

 extending over the anterior region of the eye. In one or 

 two other eyes these flattened cells are present in a less 

 definite manner over this anterior region, but more restricted 

 in extent, and not being connected with any gap. There 

 can be no doubt, I think, that this imperfect gap represents 

 the last vestige of a pupil piercing the greatl}^- developed 

 pigment layer (pars iridis), the two walls of the gap being, in 

 that case, the edges of the iris. Similar vestiges have been 

 found by Eigenmann in Amblyopsis [1, Taf. xi, fig. 9, Taf. 

 xiii, figs. 31 and 38, Taf. xiv, fig. 40], in Typhlichthys [1, 

 Taf. xiv, fig. 48], and in Troglichthys [1, Taf. xiv, figs. 54 

 and 56], and Typhlomolge [2, plate 3, figs. 6 and 7]. On 

 the other hand, the iris and lens are fairly well developed 

 in such forms as Petromyzon [6, Taf. ii, figs. 18 and 

 19], Chologaster [1, Taf. xi, fig. 4], Siphonops [6, Taf. vii, 

 fig. 67], Typhlops [6, Taf. viii, fig. 84], and Talpa [6\ Taf. 

 iii, figs. 27 and 28]. In Proteus [6, Taf. vi, fig. 59] \he lens 

 is absent in the adult, but the iris is readily distinguishable, 

 though reduced. 



lu Typhlomolge [2, pp. 53, 54] Eigenmann considers the 

 pigment filling up the pupil as being of choroidal origin. 



