274 ADAPTATIONS TO SPACE AND MOTION 



able links : an imaginative reconstruction of one extinct crossopterygian 

 fish which postulates a polymeric ring, and the situation in the extinct 

 amphibians (the Stegocephali) . 



Irregular bits of bone occur in the sclera of Triturus pyrrhogaster, and 

 the Brazilian frog Stereocyclops (=Hypopachus) incrassatus has an 

 ossified annulus around the cornea; but in no modern amphibian can any 

 certain counterparts of the sauropsidan ring be found. The stegoce- 

 phalians, however, had 'scleral ossicles'. These usually numbered 20-32 

 and were set in several rows. Moreover, they almost always formed only 

 a dorsal half -moon — rarely a closed and single ring. These bones may 

 have been homologues of the piscine circumorbitals, but it is much more 

 likely that they formed a sort of mail on the upper lid and were thus a 

 part of the head armor which was characteristic of the group and indeed 

 gave it its very name. And even if they were indeed in the sclera, they 

 could not have been involved in accommodation, for they formed only 

 part of a circle. 



The whole mechanism of accommodation which we are here calling 

 'sauropsidan' — including the scleral ossicles — may really have been in- 

 vented by the stegocephalians, which were certainly diurnal and may 

 have had sufficiently acute vision to make an excellent accommodation 

 worth while. But if so, the right stegocephalian for showing the origin 

 of the ossicular ring has not yet been found fossil. It may be significant 

 that of the cotylosaurs, the stem-group of the reptiles, no specimen has 

 yet turned up showing scleral ossicles. 



The scleral bones number sixteen or seventeen in Sphenodon. They 

 were lacking in Pleurosaurus, the largest aquatic rhynchocephalian rela- 

 tive of Sphenodon, thus affording an interesting comparison with the 

 modern crocodilians in which the ossicles probably disappeared upon the 

 advent of nocturnality, with its crude images and consequent uselessness 

 of accommodation. The Mesozoic marine crocodiles had them — at least 

 in the sidewise-looking Metriorhynchidae and in Pelagosaurus, the one 

 member of the Telosauridae whose eyes were not directed upward. 



Modern reptiles and birds have fourteen plates more often than other 

 numbers. Fourteen are usual for lizards, though there may be as many 

 as sixteen or as few (Chamaleo) as eleven. Turtles have still lower num- 

 bers — Konig found from six to nine in Testudo grceca, ten in Emys 

 orbicularis. Birds have up to eighteen, the passerines having fourteen. 

 Phylogenetic schemes based upon ossicular numbers have been attempt- 

 ed, but unsuccessfully. 



