THE SKELETON IN FISHES. VERTEBRAL COLUMN. 113 



segmentation are found in the neural and haemal arches. The 

 case is somewhat similar with the Holocephali and Dipnoi. 



In the HOLOCEPHALI the nqtochord grows persistently 

 throughout life, and is of uniform diameter throughout the 

 whole body except in the cervical region and in the gradually 

 tapering tail. The chordal sheath is very thick and includes a 

 well-marked zone of calcification which separates an outer zone of 

 hyaline cartilage from an inner zone. There are also a number 

 of cartilaginous pieces derived from the skeletogenous layer 

 which are arranged in two series, a dorsal series forming the 

 neural arches and a ventral series forming the haemal arches. 

 These do not, except in the cervical region, meet one another 

 laterally round the notochord and form centra. To each 

 neuromere there occur a pair of basidorsals, a pair of inter- 

 dorsals, and one or two supradorsals. In the tail the arrange- 

 ment is irregular. 



In the DIPNOI as in the Holocephali the notochord grows 

 persistently and uniformly, and the chordal sheath is thick 

 and cartilaginous though there are no metamerically arranged 

 centra. The neural and haemal arches and spines are cartila- 

 ginous and interbasalia (intercalary pieces) are present. The 

 basidorsalia and basiventralia do not in Ceratodus meet 

 round' the notochord and enclose it except in the anterior part 

 of the cervical and posterior part of the caudal region. 



In ELASMOBRANCHII the chordal sheath is weak and the 

 skeletogenous layer strong. Biconcave cartilaginous vertebrae 

 are developed, and as is the case in most fishes, constrict the 

 notochord vertebrally. 



Two distinct types of vertebral column can be distinguished 

 in Elasmobranchs ! : 



1. In many extinct forms and in the living Notidanidae, 



1 C. Hasse, Das naturliche System der Elasmobranchier auf Grundlage 

 des Baues und der Entwickelung ihrer Wirbelsdule, Jena, 1879 and 1885, 

 and "Die fossilen Wirbel, Morph. Studieu i. iv.," Morphol. Jahrb. Bd. n. 

 in. and iv. 1876 78. 



R. 8 



