lOO VERTEBRATE SKELETON 



only a hyomandibular suspensor, the pterygoquadrate being as truly 

 the upper jaw as in any shark. Two or three bones occur in this 

 jaw, one regarded as an entopterygoid (mesopterygoid) and a second 

 doubtfully as a palatine, while some species have a metapterygoid of 

 cartilage origin. The occludent border of the upper jaw is formed 

 by a single maxilla on either side, which articulates with a second 

 bone of doubtful homology, called both quadrate and preoperculum, 

 but probably is neither. Meckel's cartilage is enveloped in a dentale, 

 except behind where it articulates with the pterygoquadrate. The 

 hyoid arch has passed beyond the Elasmobranch in the differentia- 

 tion of a symplectic cartilage, articulating 

 with both hyomandibula and the hinder 

 end of the pterygoquadrate, the hyoman- 

 dibula being connected with the postorbital 

 process by ligament, and to the hyale 

 Firr^^ws of .4 .//>.«- by an interhyal cartilage. The rest of 

 ser. pi, pterygoid bone; d, ^i^q visccral skeleton (fig. 102) calls for no 



dentale; ?n, maxilla; ?, bone .1 , 1 r . 1 1 . 



called both quadrate and pp- remark, exccpt that several of the elements 

 ercuiare; cartilage stippled. ^^^ enveloped in ectochondrosteal bones, 

 and the first and second gill-arches are connected with the 

 cranium. 



PolyodoH has fewer bones than the sturgeons, the basis of the account above, 

 and these are more difficult to homologise. Parietals, frontals, vomers and 

 parasphenoid are evident; not so the others. The dorsal surface of the rostrum 

 is covered with small ossicles, recalling similar bones on the snout of the 

 garpike (infra). There is an anterior fontanelle and the only cartilage bones are 

 those called pre- and opisthotic. The jaws have maxilla; and dentalia, with a 

 mentomeckeHan at the tip of the lower jaw. The ossifications of the other 

 arches are like those of sturgeons, but are smaller. 



Numerous extinct Ganoids aUied to hving Chondrostei have characters 

 which make it probable that sturgeon and paddlefish of today are descendants of 

 forms with crania more like those of normal Teleostomes. Details must be 

 sought in paleontological textbooks. 



Crossopterygii. — Polypterus and Calamoichthys of African 

 rivers the only living Crossopterygians, are usually considered as 

 relatives of a number of fossils ranging from Devonian to the cretace- 

 ous, but these affinities have been questioned lately. For convenience 

 living and fossil forms are considered together. 



The chondrocranium of Polypterus only is known (figs. 108, 109, .-1). It is 

 platybasic, the cranial cavity extending to the ethmoid region. The large 



