xin PHYLUM CHORDATA 231 



smaller towards the tail. The limbs have a characteristic shape, 

 being in the form of two pairs of elongated, leaf-like, pointed 

 paddles. The mouth is situated on the ventral surface of the 

 head, close to the anterior extremity of the snout. The external 

 nares differ from those of other Vertebrates in being situated 

 immediately outside the aperture of the mouth, enclosed within 

 the upper lip. A pair of internal nares opens not far behind 

 them into the anterior part of the mouth cavity. At the root of 

 the tail is the cloacal aperture. There is an operculum similar 

 to that of the Teleostomi, with a single slit-like branchial aperture 

 behind it. 



Endoskeleton. The spinal column (Fig. 854) is represented by 

 a persistent notochord enclosed in a sheath without any trace of 



FIG. 854. Ceratodus Forsteri. Lateral view of the anterior portion of the skeleton. 

 A, anterior median membrane-bone of the roof of the skull. B, posterior median membrane- 

 bone, bas. basal cartilage of the pectoral fin ; br. branchial arches ; int. inter-operculum ; 

 lam. plate overhanging branchial region ; mck. Meckel's cartilage ; occ. rb. occipital rib ; op. 

 operculum ; pal. palato-quadrate ; pet. pectoral arch ; rbs. ribs ; sub. orb. sub-orbital bones ; 

 sq. squamosal ; supra-sc. supra-scapula. 



separate vertebrae, except in the caudal region, the segmentation 

 being indicated by the metamerically arranged neural arches and 

 ribs. Each neural arch, composed partly of cartilage, partly of 

 bone, bears on its summit a slender rod composed of three segments 

 representing a neural spine, a basal cartilage, and a radial cartilage 

 the two last extending into the unpaired fin. In the caudal 

 region the hsemal arches present a similar arrangement. The 

 most anterior of the vertebrae are. coalescent with one another and 

 with the skull. At the sides of the pne-caudal region are a series 

 of rod-like cartilages of the nature of ribs. 



The skull (Figs. 854, 855 and 856) consists of an undivided mass 

 of cartilage, narrowest between the orbits and broadening before and 

 behind ; posteriorly it is prolonged into a plate (lam.) overhanging 



