Of all the genera of _Batoidei_, Rays have the widest geographical range; they are chiefly inhabitants of temperate seas, and much more numerous in those of the Northern than of the Southern Hemisphere. They advance more closely to the Arctic and Antarctic circles than any other member of this group. More than thirty species are known, of which the following are found on the British coast:--The Thornback (_R. clavata_), the Homelyn Ray (_R. maculata_), the Starry Ray (_R. radiata_), the Sandy Ray (_R. circularis_), the common Skate (_R. batis_), the Burton Skate (_R. marginata_), and the Shagreen Skate (_R. fullonica_). Some of these species, especially the Skates, attain a considerable size, the disk measuring six and even seven feet across. All are eatable, and some of them regularly brought to market. In the majority of the species peculiar sexual differences have been observed. In some, as in the Thornback, all or some of the teeth are pointed in the male sex, whilst they are obtuse and flat in the female. The males of all are armed with patches of claw-like spines, retractile in grooves of the integument, and serially arranged occupying a space on the upper side of the pectoral fin near the angle of the disk, and frequently also the sides of the head. In species which are armed with bucklers or asperities it is the female which is principally provided with these dermal productions, the male being entirely or nearly smooth. Also the colour is frequently different in the two sexes. Other genera of this family are _Psammobatis_, _Sympterygia_, and _Platyrhina_. Although probably this family was well represented in cretaceous and tertiary formations, the remains found hitherto are comparatively few. _Arthropterus_, from the Lias, seems to have been a true Ray; and dermal spines of a species allied to the Thornback (_Raja antiqua_) are abundant in the crag deposits of Suffolk and Norfolk. FIFTH FAMILY--TRYGONIDÆ. _The pectoral fins are uninterruptedly continued to, and confluent at, the extremity of the snout. Tail long and slender, without lateral longitudinal folds; vertical fins none, or imperfectly developed, often replaced by a strong serrated spine._