VERTEBRAL COLUMN. 65 



being composed of very elastic materials, renders the 



existence of joints and vertebral bodies unnecessary. 



0. C. 235. Himterian. 



v. Ebner, Site. d. k. Akad. Wiss. Wien, Bd. civ. Hft. 3, 



1895, p. 149. 



CHORD ACENTRO us. 



B. 78. Portions of the precaudal cartilaginous vertebral column 

 of a Greenland Shark (Lwmargus borealis) , part in section. 

 Each vertebra consists of cartilage only. A disc having a 

 thickness at its centre of 4 mm., increased to 10 mm. at the 

 lateral margins, and dorsally enlarging so as to come in 

 contact with those of neighbouring vertebrae, but ventrally 

 separated by prolongations of the inter ventral ia, represents 

 the centrum. It is formed in the sheath of the notochord 

 ( = chorda-central type). Continuous with the dorsal sur- 

 face of the centrum are a pair of basidorsalia that commence 

 by a broad base. The intervals between the basidorsalia 

 are filled by the interdorsalia. Lateral processes, the basi- 

 ventralia and interventralia, largely repeat the features of 

 the dorsalia. The pairs of dorsalia and ventralia both fuse 

 in the mid line. The notochord is largely persistent. 

 Hasse, Nat. Syst. Elasmobranchier, 1882, p. 55. 



B. 79. Portions of the vertebral column of the same fish from the 

 junction of the caudal and precaudal regions. There is no 

 trace of calcification. Red glass rods indicate some of the 

 openings for the roots of the spinal nerves. 



B. 80. Portions of the vertebral column of a Fox Shark 

 (Alopecias vulpes) taken from the precaudal region. 



In the lower specimen the anterior vertebrae are in 

 sagittal section; the posterior vertebrae show the superficial 

 gtriations upon the centra due to the edges of a series of 

 radiating calcified plates, the relative size and position 

 of the arcual cartilages is also shown. 



The upper specimen is a transverse section through the 

 middle of a centrum, and shows the radiating calcified 

 plates and the relations of the bases of the arches to the 

 centrum. 



F 



