In addition to these we may briefly refer to two of the Blue Sharks (family _Carchariidæ_) that frequent our shores, distinguished by their long and prominent muzzle, and the crescent-shaped mouth. They may be regarded as higher in the scale of fish life, as compared with the sharks and rays previously named, because the vertebræ are more or less hardened by the deposit of calcareous matter, and, therefore, make a nearer approach to the character of true bone. The species referred to are the Common Blue Shark (_Carcharius glaucus_), and the Smooth Hound (_Mustelus lævis_). The former often exceeds twelve feet in length, and is commonly seen off our south and west coasts during the summer months. It is a nocturnal marauder, and is said to sleep at the surface by day with its tail exposed above the water. The Smooth Hound is a bottom feeder, subsisting on molluscs and crustaceans, the shells of which are easily crushed by its flat and blunt teeth. It is a small shark, measuring only three or four feet in length, and brings forth its young alive. [Illustration: FIG. 231.--THE SMOOTH HOUND] The next division (_Teleostomi_) contains all the bony fishes, which may be distinguished generally from the cartilaginous group by the following features:--The skeleton is more or less hardened by the deposit of calcareous matter, and the tail is generally not of the heterocercal type. The paired fins are fan-like, and the pectoral girdle is attached to the hinder part of the skull. These fishes generally have an air-bladder, and the gills lie close together in a cavity covered by an operculum. The eggs, too, are generally very small and numerous, and massed together.