'ADIPOSE LIDS' 381 



ally require them to be. The cornea is not flat as in teleosts, since the 

 elasmobranch lens must have room to move forward in accomjnodation; 

 and the outline of the cornea is often involved in the ellipticity, being 

 then much broader than it is tall. Practically never — Lamna is an excep- 

 tion — is the vertical diameter of the cornea at all greater than the 

 horizontal. The cornea is thin centrally, and markedly thickened toward 

 its rim (Fig. 104), a construction which makes of it a more sturdy dome 

 than it would be if it were uniformly thick, and also leaves more room 

 for the lens to increase its distance from the retina. Among the chon- 

 drosteans and holosteans, the shark-like sturgec^ns and the gars (which 

 make swift dashes after their prey) have ellipsoidal eyeballs. Not so the 

 slow-swimming Amia. 



In the teleosts a pronounced bulbar ellipsoidality is common, and the 

 cornea is often more or less oblong horizontally as well. Characteristically, 

 the piscine sclera consists largely of a cartilaginous cup, which is often 

 calcified (and, in Tetragonopterus, is entirely bony). In many teleosts, 

 additional support for the anterior part of the sclera is afforded by a 

 pair of osseous demilunes, disposed nasally and temporally around the 

 cornea (Fig. 130b), and sometimes fused above and below into a con- 

 tinuous ring. These demilunar ossicles are embedded in the connective 

 tissue of the sclera, and are best developed in the swiftest swimmers. 

 They are heaviest of all in the tuna (Thunnus) and the swordfish 

 (Xiphias), where they form a complete, deep 'napkin-ring' enclosing 

 nearly the whole of the eyeball (Fig. 130c). These ossicles have nothing 

 to do with the imbricated scleral ossicles of the Sauropsida (see p. 271), 

 which are homologous with the circumorbital bones of fishes. The demi- 

 lunes of modern teleosts probably represent the anterior and posterior 

 members of a quartet of ossicles which, in some of the oldest of fossil 

 fishes, formed a complete circumcorneal ring (Fig. 130a). 



* Adipose Lids'— In addition to the streamlining effect of an ellipsoidal 

 cornea, supported by scleral ossicles, many swift teleosts possess so-called 

 adipose lids, whose effect is to cover the circumocular sulcus and thus 

 eliminate distortive eddies in the slipstream alongside the eyes (Figs. 

 131a and 132). 



Considering their usual orientation, these lids are better called vertical 

 lids, for they are rarely truly adipose. Mugil cephalus forms an exception 

 — here, the lids are usually puffy, and may contain so much lipid sub- 

 stance that they turn yellow and opaque in preserved specimens. Ordin- 

 arily the vertical lids are very thin, and are perfectly transparent where 



