64 PHYSIOLOGICAL SERIES. 



to the circumference of the ring. Two isolated rings are 

 mounted above, and below are two pieces (one in section) 

 composed of several contiguous rings. 



Hasse, 'Naturliches System der Elasmobranchier/ Jena, 

 1882, p. 33. 



B. 73. Transverse sections of the spine of the Southern Chimsera 

 (CallorJiynehus australis), in which the notochord is persis- 

 tent, but has a thicker fibrous sheath, in which there is no 

 trace of calcification : the vertebrae are indicated by the 

 small cartilaginous dorsalia and ventralia. 0. C. 236 A. 

 Hasse, Nat. Syst. Elasmobranchier, 1882, p. 30. 



B. 74. A portion of the notochord of a Sturgeon. A longi- 

 tudinal section has been made on one side, which exposes 

 some small cavities in the centre. 0. C. 232. Hunterian. 



B. 75. A longitudinal section of the notochord and sheath of a 

 Sturgeon, showing the thickness of the sheath, and exposing 

 a larger central cavity in the notochord. 0. C. 233. 



Hunterian. 



B. 76. A transverse section of the spine of a Sturgeon (Acipenser 

 sturio). It shows the persistent continuous notochord : the 

 inner layer of the fibrous sheath of the notochord has 

 increased in thickness. In the skeletogenous tissue are de- 

 veloped distinct, firm, and opaque cartilages the dorsalia, 

 which consist of two superimposed pieces on each side, the 

 basal portion bounding the neural canal, the apical portion a 

 superior canal, filled by fibrous elastic ligament and adipose 

 tissue : above this is the single cartilaginous supradorsal. 

 The ventralia are now distinctly developed, and joined 

 together by a continuous expanded base, forming an 

 inverted arch beneath the notochord, for the vascular 

 trunks. 0. C. 234. Hunterian. 



B. 77. A longitudinal section of the anterior part of the spine of 

 a Sturgeon, which shows the gradual contraction of the 

 notochord as it approaches the head. The whole spine 



