72 MORPHOLOGICAL TYPES 



pectoral girdle is connected with the skull. There is no 

 clavicle, though this bone is present in more primitive bony 

 fish. 



Pelvic Girdle. — The pelvic girdle is in the form of a pair 

 of Y-shaped bones lying in the body- wall. The anterior 

 forks of the Y of the bones on each side are joined by a median 

 cartilage. The pelvic fins articulate with the outer side of 

 these bones. The radials in the pelvic fins are even more 

 reduced than those in the pectorals. 



Vertebral Column. — The centra of the vertebral column 

 are bony discs, concave on both sides, and the vertebrae 

 articulate with one another by means of facets or zygapophyses. 

 Dorsally, each centrum bears a pair of processes which join to 

 form the neural arch. This arch is produced farther into the 

 neural spine. The radial surmounts this and, in the regions 

 of the dorsal fins, is articulated with the lepidotrichia. The 

 radials scarcely project at all into the fins. The spinal nerves 

 emerge between the neural arches. 



In the trunk-region, each centrum bears a pair of ventro- 

 lateral processes, to which the (" ventral," see p. 83) ribs are 

 attached. In the region of the tail these processes are directed 

 downwards, and join to form the haemal arches. These are 

 prolonged into the haemal spines, which support the ventral 

 lobe of the tail-fin. The neural arches correspond to basi- 

 dorsals, and the ventro-lateral processes to basiventrals. The 

 notochord is of course obliterated by the centra. 



Before leaving the skeleton, it remains to sort out the 

 various bones into cartilage-bones and membrane-bones, and 

 those whose constituents arise in both ways and which may 

 therefore be called mixed bones. 



