620 REPTILES 



reptiles) , and TxOt parts of the underlying anterior-epithelial cells as are 

 the dilatator elements of mammals. Both retinal layers (as in lizards) 

 are heavily pigmented — the anterior, even more so than the posterior 

 (as in all vertebrates in which a dilatator is lacking or, if present, is not 

 formed as a lamina of the anterior retinal layer) . 



The anterior chamber is very shallow and the lens is large — 8.0mm. 

 X 6.33mm. — in keeping with the nocturnal habits of the animal. Thus, 

 the quotient of eye diameter and lens diameter (17/8) is 2.12 in Spheno- 

 don, 2.7 in Iguana, and 2.78 in Uromastix icf. corneal proportions, 

 above). The flatness-index of the lens (8/6.33) is 1.26, while the lenses 

 of diurnal lizards average somewhat flatter (1.4-1.5) and that of a 

 terrestrial turtle (Testudo grceca) is flatter still (1.6). The alligator 

 lens is about as rotund as that of Sphenodon, however (1.25), and 

 nocturnal lizards (geckoes) have nearly spherical lenses (e.g., Tarentola 

 mauretanica — l.l). The Sphenodon eye may perhaps have a relatively 

 large retinal image for a nocturnal animal; but this is not out of line 

 with its retention of other features having to do with the maintenance 

 of good resolution — the fovea, for instance, as well as some of the saur- 

 opsidan adjuncts to good accommodation (scleral ossicles, ringwulst). 

 The anterior surface of the lens is much less sharply curved than the 

 posterior, and the ringwulst is well developed, its thickness being 6% 

 of the diameter of the whole lens. 



There are no vitreal vessels; and there is no conus papillaris — and 

 even less trace of one than in the crocodilians, for the optic disc is not 

 even convex, and shows only a very few melanin granules. The disc is 

 slightly temporal and considerably ventral in position, its center lying 

 about 2.5mm. from that of the exactly (?) central fovea. The sensory 

 retina, as in large lizard eyes, tapers gradually in thickness anteriorly, 

 so that the ora is not abrupt. The optic nerve is relatively slender, with 

 a simple circular cross-section, and entirely lacks any septal system. Else- 

 where in the reptiles, so simple a situation occurs only in the (also 

 nocturnal) crocodilians. 



The Retina — Because of the coarseness of its visual-cell mosaic and 

 its Miiller fibers (which become massive in the far periphery, and occupy 

 most of the volume of the retina there), the Sphenodon retina appears 

 at first glance to resemble that of the turtles. Closer analysis shows that 

 the strongest similarities are to the lizards. Sphenodon has a concavi- 

 clivate fovea (Fig. 82, p. 189), which in the diurnal ancestor was prob- 

 ably entirely lizard-like. 



