xin PHYLUM CHORDATA 233 



pair of investing bones. 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 expanse of the fin. The pelvic arch is 

 a single cartilage, produced forwards into an elongated rod-like 

 process. The skeleton of the pelvic fin (Fig. 857) is similar to 

 that of the pectoral. 



Digestive Organs. The teeth (Fig. 856) are of a remark- 

 able and characteristic shape. There are two pairs of large 

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

 roof of the mouth (palato-pterygoid region) and the other 

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

 border, which is directed inwards and somewhat backwards, 

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



FIG. 857. Ceratodus Forsteri. Pelvic arch and skeleton of pelvic fin. (After Giinther. 



bluntly pointed 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 (d') placed close together and 

 directed vertically. 



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 true 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 

 hyoidean gill or pseudobranch is present as well. The lung (Fig. 858) 

 is an elongated median sac connected with the ventral wall of the 



