XII 



PHYLUM CHORDATA 



397 



vertebrae are present, the centra of which are nearly always 

 bi-concave. 



In the Sturgeons (Fig. 224) and their allies the cranium 

 is an undivided mass of cartilage with a few isolated 

 cartilage 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 



Me/ 



FIG. 224. Skull of Sturgeon, with the membrane bones removed, a. pharyngo- 

 branchial ; AF. antorbital piocess ; AR. articular ; b. epibranchial ; c. cerato- 

 branchial ; C. notochord ; Cop. basi-branchials J if. hypobranchial ; De. dentary', 

 GK. auditory capsule ; Htn. hyomandibular ; hy. hyoid cornu ; Ih. inter-hyal ; 

 Md^ mandible ; No., nasal capsule ; Ob. neural arches ; PF. post-orbital process ; 

 PQ. palato-quadrate ; Ps. Ps '. Ps". parasphenoid ; Psp. neural spines ; Qu. 

 quadrate ; A', rostrum ; Ri, ribs ; Sp. N f foramina for spinal nerves ; Sy. 

 symplectic ; WS. vertebral column ; A. vagus foramen ; IV. branchial arches. 

 (From Wiedersheim's Comparative Anatomy.) 



membrane bones (Fig. 225). Both upper and lower jaws 

 are bounded by membrane bones (p.mx, mx, dent). The 

 jaws are connected with the skull by the intermediation of 

 a hyomandibular (hyoiti). The ectoral arch is complicated 

 by the addition 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 



