400 



MANUAL OF ZOOLOGY 



In the sturgeons (Fig. 240) and their allies the cranium 

 is an undivided mass of cartilage with a few isolated carti- 

 lage bones, and covered over dorsally by membrane bones. 

 In most of the other members of the group it is mainly or 

 entirely composed of numerous cartilage and membrane 

 bones (Fig. 240). Both upper and lower jaws are bounded 

 by membrane bones (/. mx, mx, dent). The jaws are con- 

 nected with the skull by the intermediation of a hyomandib- 



FlG. 240. — Skull of sturgeon, with the membrane bones removed, a, pharyngo- 

 branchial; AF, antorbital process; AR, articular; b, epibrancbial ; c, cerato- 

 brancbial; C, notochord ; Cop, basi-branchials; d, hypobranchial; De, dentary; 

 GK~, auditory capsule; HM, hyomandibular; hy, hyoid cornu; //t, inter-hyal; 

 Aid, mandible; Na, nasal capsule; Gb, neural arches; PF, post-orbital pro- 

 cess; PQ, palato-quadrate; Ps. Ps' . Ps" , parasphenoid; Ps/>, neural spines; 

 Qu, quadrate; R, rostrum; AY, ribs; Sp. .A 7 ", foramina for spinal nerves; Sy, 

 symplectic; }l'S, vertebral column; «, vagus foramen; /-/", branchial arches. 

 (From Wiedersheim's Comparative Anatomy.) 



ular (hyom) which, however, probably does not correspond 

 with the cartilage so named in the dogfish and other Elas- 

 mobranchs. The pectoral arch is complicated by the addi- 

 tion of membrane bones, of which the most constant are a 

 pair of large clavicles. The pelvic arch is vestigial or absent. 

 Two genera of Teleostomi possess electric organs, — the 

 electric catfish (Alalapterurus) and the electric eel ( Gym- 

 notus) ; the former occurs in fresh waters of tropical Africa, 



