250 



ZOOLOGY 



SECT. 



The latter are connected with the skull by post-temporals. The 

 skeleton of the pectoral fin consists of a stout basal cartilage (bos.), 

 an elongated, tapering, central axis made up of a number of short 

 cartilaginous segments, and two rows of jointed cartilaginous rays 

 extending out on either side of the axis so as to support the 

 middle part of the expanse of the fin. The pelvic arch is a single 

 cartilage, produced forwards into an elongated rod-like epipubic 

 process (Fig. 907). The skeleton of the pelvic fin is similar to 

 that of the pectoral. 



Digestive Organs. The teeth (Fig. 906) are of a remark- 

 able and characteristic shape. There are two pairs of large 

 compound teeth of similar character, one pair (the palatine, d), on 

 the roof of the mouth (palatopterygoid bone) and the other 

 (splenial) on the lower jaw. Each is a curved plate with the convex 

 border, which is directed inwards and somewhat backwards, 



FIG. 007. Ceratodus forsteri. Pelvic arch and skeleton of pelvic fin. (After Gttnther.) 



entire ; while the concave border presents a series of six or seven 

 vertical, ridge-like projections or cusps. In addition to these, there 

 are, in front of the palatine pair, a pair of much smaller, simple, 

 somewhat chisel-like vomerine teeth (cT) placed close together and 

 directed vertically. In the embryo each tooth is represented by a 

 number of separate denticles which subsequently coalesce. 



In the enteric canal the chief feature of special interest is the 

 presence, throughout the length of the intestine, of a spiral valve 

 similar to that of the Elasmobranchs and Ganoids. The rectum 

 opens into a small cloaca. A pair of abdominal pores open just 

 behind this. 



Organs of Respiration. Ceratodus combines aquatic respira- 

 tion by means of gills similar to those of ordinary fishes, with 

 aerial respiration by means of a lung. 



There are four pairs of gills, each consisting of a double row of 

 gill-filaments supported on the branchial arches. A rudimentary 



