CHONDROSTEI. 



169 



maxilla and dentary) are developed in connection with the jaws. 

 Teeth are entirely absent in the adult, but small teeth are present 

 in the young Polyodon and possibly other genera. There are 

 five branchial arches. 



In the unpaired fins the fin rays (dermotrichia) are more 

 numerous than the interspinous supports (somactids), and the 

 caudal fin has fulcra in a single row. The pectoral arch is un- 

 ossified and the two halves are not united ventrally. They are 

 covered by membrane bones; supra-clavicles, clavicles, and infra- 

 clavicles being present. The pelvic arch is absent, its place being 

 taken by enlarged basal cartilages of the fins as in Teleostei. 

 The paired fins are without a basal lobe (non-lobate), their whole 

 free portion being supported by dermal fin-rays. These are 

 carried by a row of cartilages of which the posterior is possibly 



Fig. 95. — Cephalic skeleton oi the sturgeon (from Glaus after Wiedersheim). Ro Rostrum ; 

 Cn nasal pit; O orbit; Ilm hyomandibular ; S symplectic ; Pq palatoquadrate ; ild 

 lower jaw ; Hy hyoid ; T' foramen for vagus ; B. ribs. 



a basal somactid (metapterygium) and the others peripheral 

 somactids which have become articulated to the pectoral arch. 

 The anterior dermal fin-ray is enlarged and directly articulates 

 with the pectoral girdle. 



The branchial apparatus presents remarkable variability. In 

 Acipenser there is a spiracle with a pseudobranch, and a hyoid 

 demibranch ; in Scaphirhynchus there is no spiracle or pseudo- 

 branch, but a hyoid demibranch is present. In Polyodon spiracle 

 and pseudobranch are present, but there is no hyoid gill. Each 

 of the first four branchial arches carries a double row of gill- 

 filaments and there is a cleft behind the fourth gill arch. In 

 Acipenser the inter-branchial septa are fairly broad. In 

 the conus there are three longitudinal rows of valves with three 

 or four valves in each row. 



