490 



FISHES 



Fishes, and also with others obviously due to degeneration. The 

 most interesting illustration of the first point is to be found in 

 the condition of the primitive upper jaw which, especially 

 in the Polyodontidae, is typically Elasmobranch in the median 

 union of the palato-quadrate bars beneath the basis cranii, but 

 Teleostome in the presence of a secondary upper jaw formed by 

 two maxillae. Both families also agree in possessing an acentrous 

 vertebral column which, if it does so far resemble that of Teleo- 

 stomes in being potentially arco-centrous, nevertheless has a 

 better developed series of distinct inter-dorsal and inter -ventral 

 cartilages, regularly alternating with only partially bony basi- 



FIG. 288. Lateral view of a restored skull and pectoral girdle of Chondrosteus acipen- 

 seroides. a, Angular ; br, branchiostegal rays ; c.h, cerato-hyal ; h.m, hyomandi- 

 bular ; j, jugal ; p.f, post-frontal ; s.op, suboperculum ; s.t, supra-temporal ; other 

 reference letters as in Fig. 284. (After Traquair.) 



dorsals and basi-ventrals, than is to be met with in any other adult 

 Fishes except Elasmobranchs. Primitive features are apparent 

 in the presence of spiracles, sometimes associated with pseudo- 

 branchs ; the presence in one family (Acipenseridae) of a 

 hyoidean hemibranch supplied with blood directly from the 

 ventral aorta, and the existence of a multi-valvular conus 

 arteriosus and an intestinal spiral valve. Finally, the massive 

 growth of the chondrocranium wholly devoid of cartilage bones, 

 except in so far as they may be represented by splint-like 

 membrane bones, the fragmentation of the investing dermal 

 bones, the degeneration of the opercular skeleton and the loss of 

 branchiostegal rays, and the almost complete disappearance of 

 the primitive rhombic squamation, are probably to be regarded 



