608 REPTILES 



the fashion of the teleostean falciform process. Of all the reptilian 

 peculiarities, only the transversalis muscle may be homologous with any- 

 thing in the living amphibians (i.e., with their ventral protractor lentis). 

 The early reptiles adopted strict diurnality and a pure-cone retina; but 

 many of the living forms and sub-groups have backslid into nocturnality, 

 supporting this habit with a rod-rich or even pure-rod retina whose rods 

 are transmuted cones in every case. 



To every one of the above statements the snakes constitute a conspic- 

 uous exception. There is nothing whatever 'reptilian' about their eyes, 

 which exhibit instead a number of features which are uniquely ophidian. 

 Indeed, the snake eye is such a conglomeration of 'Ersatz' that it might 

 well be imagined to have come from another world. Zoologists have long 

 been fond of citing the cephalopod molluscs, as showing how nearly an 

 invertebrate group can imitate the vertebrate eye if it tries hard (see Fig. 

 Ig, p. 3). They might give at least as much credit to the snakes; for in 

 them, we see a vertebrate group which has been under the necessity of du- 

 plicating the vertebrate eye, and has made a very good job of it. This no 

 doubt obscure statement will be clarified by the discussion in Section D. 



(A) Chelonians 



See also pages: 



59 Fig. 24c 251 Fig. 100 



72 vision 272-9, 436-8 accommodation, refraaion 



101-2 zapfensubstanz 274 scleral ossicles 



135, 138 Fig. 60, relationships 293 visual fields 



177 retina ^^^ eye movements 



184,187,190,305 area centralis, fovea ,^^ parieta eye 



344 movement-perception 

 ^^^ ^'S-^^ 422-3,428,450,457-8 adnexa 



191-8,202 oiWroplets 435^ amphibious adaptations 



216 visual cells 494.6, 519 color vision 



224 pupil 546-7 coloration of eye 



Though obviously highly specialized, the Chelonia are nevertheless 

 the most archaic of the living orders of reptiles — closest of all to the stem 

 group, the Cotylosauria. They are cosmopolitan, and comprise eleven 

 families in four suborders. The taxonomic differentiation of the group 

 is of less importance to us here than the ecological — into the strictly 

 aquatic marine 'turtles' (s.s.), the amphibious, freshwater 'terrapins', and 

 the strictly terrestrial 'tortoises' (see, especially, the references to accom- 

 modation and adnexa) . 



