Fig. 126.—Raja lemprieri, from Tasmania.
Fig. 127.—Dermal spines of a male Thornback, Raja clavata.
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.
The “Sting-Rays” are as numerous as the Rays proper, but they inhabit rather tropical than temperate seas. The species armed with a spine use it as a weapon of defence, and the wounds inflicted by it are, to man, extremely painful, and have frequently occasioned the loss of a limb. We have mentioned above (p. [190]) that the danger arises from the lacerated nature of the wound as well as from the poisonous property of the mucus inoculated. The spines (Fig. [98], p. 190) are always barbed on the sides, and may be eight or nine inches long in the larger species. They are shed from time to time, and replaced by others growing behind the one in function, as the teeth of the fishes of this order, or as the fangs of a poisonous snake. Fossil species of Trygon and Urolophus occur in the tertiary strata of Monte Bolca and Monte Postale. The genera into which the various species have been divided are the following:—
Urogymnus.—Tail long, without fin or spine, sometimes with a narrow cutaneous fold below. Body densely covered with osseous tubercles. Teeth flattened.