ELASMOBRANCHIL 373 



transverse, and is placed on the under surface of the head (fig. 

 141, A). The exoskeleton consists of placoid granules, tuber- 

 cles, or spines. The branchial sacs open externally by as 

 many distinct apertures as there are sacs, and there is no oper- 

 culum. A pair of tubes proceed from the pharynx to open 

 on the upper surface of the head by two apertures, which are 

 termed " spiracles." By means of these water can be admitted 

 to the pharynx, and thence to the gills. 



By Professor Owen the Plagiostomi are divided into three 

 sections, termed respectively the Cestraphori, the Selachii, and 

 the Batides. 



a. Cestraphori. In this division there is a strong spine in 

 front of each dorsal fin, and the back teeth are obtuse. The 

 only living representative of this group is the Port Jackson 

 Shark (Cestradon Philippt), characterised by its pavement of 

 plate-like crushing teeth, adapted for comminuting small Mol- 

 luscs and Crustaceans. It is exclusively an inhabitant of the 

 Australian seas, and is remarkable for its close resemblance to 

 a large group of extinct forms, of which the best known are 

 the genera Hybodus and Acrodus from the Secondary Rocks. 



b. Selachii. This group comprises the formidable Sharks 

 and Dog-fishes, and is characterised by the lateral position of 

 the branchiae on tKe side of the neck, and by the fact that the 

 pectoral fins have their ordinary form and position. The Dog- 

 fishes are of common occurrence in British seas, but are of 

 little value. Their egg-cases are frequently cast up on our 

 shores, and are familiarly known as " Mermaid's purses." The 

 true Sharks are not infrequently found in various European 

 seas, but they are mostly inhabitants of warmer waters. One 

 of the largest is the "White Shark" (Carcharias vulgaris\ 

 which attains a length of over thirty feet (fig. 142, A). 



c. Batides. This group includes the Rays and Skates, and 

 is distinguished by the fact that the branchial apertures are 

 placed on the under surface of the body, forming two rows of 

 openings a little behind the mouth. In the typical members 

 of the group, the body is flattened out so as to form a kind of 

 disc (fig. 143), the greater part of which is made up of the 

 enormously developed pectoral fins. Upon the upper surface 

 of the disc are the eyes and spiracles ; upon the lower surface 

 are the nostrils, mouth, and branchial apertures. The flat- 

 tened bodies of the Rays, however, must be carefully distin- 

 guished from those of the Flat-fishes (Pleuronectida). In the 

 former, the flat surfaces of the body are truly the dorsal and 

 ventral surfaces. In the latter, as before remarked, the body 

 is flattened, not from above downwards, but from side to side, 



