130 FREDERICK TILNEY AND LUTHER F. WARREN 



ganglion and passed close behind the first nerve to the parietal 

 eye. The lens of the parietal eye is not uniform in its shape; 

 it occurs in the following different forms: 



1. Regular bi-concave lens, both surfaces curved, which is 

 most common in Lacerta vivipara, Lacerta agilis and Lacerta 

 ocellata, Leiolaemus nitidus, Seps chalcidica, Phrynosoma doug- 

 lassi, and Sphenodon. 



2. Bi-convex, with the under surface more convexed than the 

 upper, as in Anolis and Sphenodon. 



3. Plano-convex, as in Anguis and Iguana. 



4. Concavo-convex, as in Calotes, Varanus bengalensis, and 

 Varanus giganteus. 



The structure of the lens is made up of peculiar, long, cylin- 

 drical cells apparently derived from modified ependymal cells. 

 These are the so-called lens cells. There are some intercellular 

 spaces, probably lymph spaces, according to Leydig ('91). 238 

 The lens cells are nearly free of pigment. The substance of these 

 cells is very hard. Their nuclei are oval or round and are sel- 

 dom scattered over the entire lens surface or its entire thick- 

 ness. They are most numerous at the border of 'the lens where 

 the latter passes over into the retina. 



The parietal foramen. Leydig 234 in 1872 found a round or oval 

 opening in the skull of Sphenodon situated in the osparietal, 

 which seemed either directly to serve as the outlet for the parietal 

 organ or else for the entrance of light rays. It was reminiscent 

 of a similar opening in the cartilaginous roof of the cranium in 

 selachians. In most cases the parietal eye is in, or directly 

 under, this foramen. Species which do not possess a parietal 

 eye have a parietal foramen which is filled by the pineal organ, 

 in which case, the end-vesicle takes the place of a third eye as 

 far as location is concerned. The foramen is absent in a large 

 number of saurians, particularly in the Geckonidae, and it is 

 also absent in Ceratophora aspera. There are also instances in 

 which the foramen does actually appear in some individuals of a 

 species and yet in other individuals of the same species it is closed 

 by bone. The eye usually lies in the middle of this foramen or 

 near its upper edge. The relation between eye and foramen is 



