422 MANUAL OF ZOOLOGY. 



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 (fig. 180). 



The only living representative 

 of this group is the Port Jack- 

 son Shark (Cestradon Philippi}, 

 characterised by its pavement 

 of plate-like crushing teeth, 

 adapted for comminuting small 

 Molluscs and Crustaceans. It is 

 exclusively an inhabitant of the 

 Australian seas, and is remark - 

 fr .... able for its close resemblance to 



Fig. 180. Dental plates of Cochliodus _ . 



contortus, an extinct Cestraciont a large gTOUp OI CXtlllCt forms, 

 (Carboniferous). 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 the 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. 179, A). The 

 body in the Sharks (Squalida) is not rhomboidal, but is elon- 

 gated ; the nostrils are placed on the under side of the snout, 

 and the teeth are arranged in several rows, of which the outer- 

 most alone is employed, the inner ones serving to replace the 

 former when worn out. 



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 

 rhomboidal disc (fig. 181), 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 aper- 



