SCLERAL OSSICLES OF SAUROPSIDANS 



271 



snakes — the latter also lacking the scleral cartilage, among a great many 

 other ocular structures which other sauropsidans possess. The scleral 

 ossicles are not convexly curved to continue the rotundity of the equator- 

 ial sclera smoothly into the sharper curvature of the cornea. On the con- 

 trary they are flat or even concave, so that the whole zone of the sclero- 

 corneal junction is depressed or concave to form a broad annular sulcus 

 (Fig. 107d). This sulcus is important to the fundamental processes of 

 sauropsidan accommodation. The ossicles which are responsible for it are 

 therefore considered here as a part of the mechanism of accommodation 

 in these vertebrates. 



There is little question as to the evolutionary source of these ossicles, 

 but the time of their origin is in doubt. Surrounding the eyes of fishes is 

 a ring of small skull bones, the circumorbitals. The overlying skin often 

 bears a sense organ (of the lateral line system) centered over each of the 



Fig. 108 — Sauropsidan embryos, 

 showing sensilloid papilla at pe- 

 riphery of cornea: evidence for 

 the origin of scleral ossicles from 

 extra-ocular bones (see text). 

 From Franz, after Dabelow. 



a, Lacerta agilis. b, Vanellus sp. 



circumorbital bones. In sauropsidan embryos the scleral ossicles arise as 

 dermal bones which sink into the sclera, and the ectoderm over each one 

 temporarily shows a sensilloid papilla (Fig. 108). It thus appears that 

 the scleral ossicles are homologous with the originally ex/rd-ocular cir- 

 cum-orbital bones. 



Another theory derives the sauropsidan scleral ossicles from the scleral 

 bones of fishes, which being co-existent with the circumorbital bones could 

 scarcely be homologous with the latter. Ancient armored fishes had four 

 plates in the anterior sclera, forming a closed ring. Modern fishes show 

 at most only two of these — if they are indeed homologues. In some swift 

 swimmers, the tuna and swordfish for example, the two plates are joined 

 to make a complete ring and are protective against the impact and dis- 

 tortive pressure of the water (see Fig, 130, p. 380), 



These oligomeric ossicular rings of fishes have been dubiously homo- 

 logized with the polymeric ones of the Sauropsida through two question- 



