THE ELASMOBRANCH FISHES 



47 



are tlie so-called labial cartilages (see fig. 48, 1.), a single one of which is found 

 in Heptanchus ijiaculatns. This is an irregular cartilage, interesting particu- 

 larly because of its unusual development in this species. An extravisceral is 

 present dorsally on the hyoid arch (ex.h., fig. 48), but none is present ven- 

 trally. Similar cartilages are found both dorsally and ventrally on all bran- 

 chial arches except the last. The extrabranchial cartilages (ex.h., fig. 49) 

 curve around the tips of the branchial rays as a protection and a support for 

 the outer margins of the gill septa. 



In addition to the arches above described there are, especially in some of the 

 young specimens of Heptanchus, supernumerary rudiments of still other 



hd. id sbd. 



Fig. 51 



Fig. 52 



Fig. 51. Fifth to eighth segments of the spinal column, Heptanchus maculatus. 



Fig. 52. Sagittal section through sixth to eighth segments of the column. (H.M.Gilkey, del.) 



bd., dorsal basal (basidorsal) plate; bv., ventral basal (basiventral) plate; c, central 

 column; chd., unconstrieted part of notochord; f.d., foramen for dorsal root nerve; 

 f.v., foramen for ventral root nerve; h.a., haemal arch; id., dorsal intercalary (interdorsal) 

 plate; iv., ventral intercalary piece; is., layer immediately around the gelatinous notochord; 

 m.s., middle layer in sheath of notochord ; n.c, neural canal ; oz., cartilage ; r., rib ; s., sep- 

 tum constricting notochord ; s.bd., so-called neural spine. 



branchial arches. An eighth appears directly back of the last functional arch 

 (ch.^, fig. 50a) and even a ninth arch may be indicated nearer the middle line 

 (ar.^, fig. 50b), (Daniel, 1916). 



Spinal Column 



The spinal column in Heptanchus, because of its simplicity, is especially inter- 

 esting. Unlike that of the higher Elasmobranchs, it consists of a long central 

 column {c, fig. 51) which is essentially the enlarged sheath of the notochord. 

 Anteriorly the column is more or less continuous with the occipital region of 

 the cranium (see fig. 47) and posteriorly it extends to the tip of the tail. Above 

 this central cohunn there is a series of neural arches formed for the protection 

 of the spinal cord; below it, in the region of the tail, is a similar series of 

 haemal arches {h.a., fig. 53) for the protection of the caudal artery and vein. 

 A segment of the column in the so-called neck region shows the central part 

 well developed. Above this central part are the neural plates making up the 

 neural arch. Each arch is composed of a dorsal plate (hd., fig. 51) and a dorsal 



