B Selachii. This group comprises the formidable Sharks and Dogfishes, 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, and their anterior ends are not connected with the skull by cartilages. The skull also has a median facet for the first vertebra.
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 embryo possesses external branchiae, developed both from the spiracle and the branchial arches; but these structures disappear in the course of growth. 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. The body in the Sharks (Squalidae) is not rhomboidal, but is elongated ; the nostrils are placed on the under side of the snout, and the teeth are arranged in several rows, and are in the form of compressed cones. During life, the cartilaginous jaws are so far flexible that their margins can be partially everted, thus bringing more than one row of teeth into use at one time; but the innermost rows are principally employed to replace the outer rows, when the latter are worn out.