490 FISHES CHAP. 
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- 
Fia. 288.—Lateral view of a restored skull and pectoral girdle of Chondrosteus acipen- 
seroides. a, Angular ; by, branchiostegal rays; c.h, cerato-hyal; h.m, hyomandi- 
bular ; 7, jugal ; p.f, post-frontal ; s.op, suboperculum ; s.¢, 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 
