ELABMOBRANCHIL 397 



a large portion of their borders, and there are, consequently 

 five holes communicating with the gill-slit. A rudimentary 

 operculum is present, covered by the' skin. In the closely- 

 allied Callorhynchus from the South Seas, there is a large fleshy 

 appendage at the end of the snout. In the Secondary and 

 Tertiary Rocks, are found several fossil forms, constituting the 

 genera Edaphodus, Elasmodus, and Ischiodus. 



Sub-order B. Plagiostomi. — This sub-order is of consider- 

 ably greater -importance, as it includes the well-known Sharks 

 and Rays. The vertebral centra are usually more or less ossi- 

 fied, and even when quite cartilaginous, the centra are marked 

 out by distinct rings. . The skull is in the form of a cartila- 

 ginous capsule, without distinct cranial bones. The mouth is 

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

 149, A). The exoskeleton consists oCplacoid 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 Sdachii, 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 {Cestracion 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 branchise on the side of the neck, and by die 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 attaiiis a length of over thirty feet (fig. 150, A). The 

 body in the Sharks {Squalidce) is not rhomboidalj iDut is elon- 



