PISCES. 415 



a complete investment to the spinal cord in Elasmobranchii and Holo- 

 cephali. They are differentiated into neural arches ( = crural cartilages) 

 which may or may not bear neural spines, and into intervening inter- 

 crural cartilages which are placed intervertebrally. These parts may be 

 incompletely ossified. The haemal ridges in the caudal region are 

 differentiated similarly. Small intercrural cartilages are present in Aci- 

 penser, and are perhaps represented in Lepidosteus and some Teleostei 

 (p. 101). The ossified neural and haemal arches of Amia are connected 

 to the vertebral centra by cartilage discs : in Polypterus, Lepidosteus, and 

 Teleostei they are continuous with the centra and are formed by parosteal 

 ossification round cartilage, which may or may not persist. The right 

 and left arches are connected inter se above the spinal cord by either bone, 

 cartilage, or ligament. The summits of the neural spines and of the haemal 

 spines perhaps form the supports for the dorsal and anal fins respectively 

 (p. 101). In all Fish a superior longitudinal ligament connects all the neural 

 arches above the neural canal. Bony zygapophyses are formed near the 

 bases of the neural arches in some Teleostei. A cartilaginous rod terminates 

 the backbone in Ganoidei, some Teleostei, and the Dipnoi. It is surrounded 

 by a bony sheath (urostyle) in some Teleostei, and gives support to more 

 than one neural and haemal arch. 



There are only two regions in the backbone, a dorsal and a caudal 

 the latter distinguished by the ventral fusion of the haemal arches or ribs 

 (p. 100). The largest number of vertebrae is found in Elasmobranchii. In 

 some of the latter, e.g. Scy Ilium and Raja, there are twice as many centra 

 in the tail as there are myomeres and spinal nerves. The most anterior 

 vertebrae coalesce with the skull in Ganoidei, some Teleostei and Dipnoi. 

 In some Elasmobranchii at least, the two ' structures are articulated by 

 distinct exoccipital condyles. 



The ribs are cartilaginous in Elasmobranchii, Holocephali, chondrostean 

 Ganoidei, and Dipnoi. They are always simple in form, and lie in Elasmo- 

 branchii in the fibrous septum between the dorso- and ventro-lateral 

 muscles. In other Fish they lie close to the peritoneum, but their ends 

 turn outwards in Lepidosteus into the septum named. They correspond to 

 the myocommata, in which slender bones (epi-centrals, epi-neurals, epi- 

 pleurals) may be formed in some Teleostei. 



As to the limbs, the ventral fins are absent in a few Teleostei, e.g. 

 Muraenidae, Gymnotidae, among Physostomi ; Syngnathidae among Lopho- 

 branchii ; Ostraciontidae and Gymnodonts among Plectognathi, or else repre- 

 sented by moveable spines, as in Balistidae and Triacanthidae in the order 

 last named. Their occasional absence has been noted in Protopterus. 



The shoulder-girdle is cartilaginous in Elasmobranchii, Holocephali, 

 chondrostean Ganoidei and Dipnoi, ossified as a separate scapula and 

 coracoid (claviculo-coracoid) in bony Ganoidei and Teleostei. It is of large 



