48 



THE ELASMOBRANCH FISHES 



intercalary piece (id.). 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 dorsal plate there may be pieces segmented off 

 (s.hd.) to form the so-called neural spines, and in the most anterior part of 

 the column two such pieces may be present one above the other (fig. 47) . Each 

 dorsal plate in the anterior region is further perforated by a ventral root 

 (f.v.) of the spinal nerve, and each dorsal intercalary by the dorsal root (f.d.) 

 of the same nerve. 



In this region and ventral to the central column are also ventral plates 

 (bv., fig. 51). On the third and succeeding vertebrae back to the forty-fifth, 

 ribs (r.) are present. The eighth to twenty-fourth ribs in Hepfanclius, like 

 some of those in Laemargus (Helbing, 1904, cited on p. 72), are divided into 



f.v.f.d. 



Fig. 53. Lateral view of spinal column in transitional area. Drawai as transparent object. 

 (Katharine Rogers, orig.) (For explanation see fig. 52.) 



an anterior and a posterior part, the former of which is a curious plate-like 

 process projecting forward and downward. Between two ventral plates there 

 is interpolated a small ventral intercalary piece (iv.). 



A sagittal section through this region (fig. 52) shows the central column 

 composed of three concentric layers in the notochordal sheath. These layers 

 surround the notochord (chd.) and constrict it at intervals into a bead-like 

 chain. The outermost of these layers is relatively thin and consists of carti- 

 lage; within this cartilage is a second and lighter broad area (m.z., fig. 52) 

 which appears to be made up of transverse fibers. Within this second layer 

 and bounding the notochord is a third layer (is.) of a white tissue. At regular 

 intervals the third layer forms septa (s.) which produce the regular constric- 

 tions in the central part of the notochord. It will be observed that the septa are 

 more pronounced ventrally than dorsally and that they pass intraeentrally. 



In the midbody the central column assumes its simplest form. Here it con- 

 sists essentially of a heavy and but slightly constricted sheath which if it be 

 allowed to dry slightly gives greater evidence of constrictions. At about the 

 fiftieth segment of this region (fig. 53) some of the dorsal plates extend en- 

 tirely to the top of the arch so that no neural spines are present. Following 

 these plates, and beginning at about the fifty-sixth segment two types of dorsal 

 plates obtain, one of which is high, the other much lower. The higher is per- 

 forated by the ventral root of the nerve; the lower is imperforate. The higher 

 plate is followed by a dorsal intercalary plate, perforated by the foramen for 

 the dorsal root nerve. In the region beyond the fifty-fifth segment it will be 



