358 University of California Publications in Zoology V^'o^- 16 



Over all of the branchial arches excepting the last these cartilages 

 are found both dorsally and ventrally. On the anterior branchial 

 arch (ex.b., text-fig. A) they attain a large size, vet even here they do 

 not overlap as in Heterodontus. They curve laterally around the tips 

 of the branchial rays and serve to protect the underlying structure 

 and to support the septa. 



The Spikal Column 



The spinal column in Heptanchus, because of its generalized char- 

 acter, is especially interesting. It consists of a long central column 

 (c.) which anteriorly is more or less indistinguishable from the occipi- 

 tal region of the cranium (see pi. 28, fig. 3) and which in the body 

 region is undivided into separate centra (text-fig. P). If the column 

 is allowed to dry slightly, differentiation of this central column into 

 segments may be made out. The septa which produce this segmenta- 

 tion of the colunm, however, run through what would be the middle 

 of a centrum in a more specialized form. A series of dorsal arches 

 protecting the spinal cord extends practically the whole length above 

 this central column, and a series of haemal arches is present ventral 

 to it in the region of the tail (h.a., text-fig. F). 



For convenience of study we may divide the column into regions : 

 a section in the so-called cervical region shows the segments (c, text- 

 fig. D, and pi. 28, fig. 3) relatively well developed. Above these are 

 the plates making up the neural arch, each arch being composed of 

 a basiclorsal {bd.) and an interdorsal (id.) plate. Both of these 

 cartilages are more or less triangular in shape, the former having its 

 base on the centrum, the latter with apex pointing toward the centrum. 

 Above the basidorsal there is a piece segmented off as the suprabasi- 

 dorsal (s.bd.) and in the anteriormost part of the column two such 

 pieces are formed, one above the other (fig. 2). Each basidorsal is 

 further perforated by the ventral root (/.c.) of a spinal nerve, and 

 each interdorsal is pierced by the dorsal ramus (/.d.) of a spinal 

 nerve. Ventral to the central colunm are also basiventral pieces (bv.) 

 between which may be interpolated small interventrals (iv.). From 

 the third and succeeding basiventrals back to about fhe forty-fourth, 

 ribs (r.) are formed. Beginning with the eighth vertebra and con- 

 tinuing back in cases to the twenty-fourth the ribs in Heptanchus 

 maciilatus, like those in Laemargus. are divided into an anterior and 

 a posterior part, the former of which is a curious plate-like process 

 projecting forward and downward. 



