STING-RA VS. 



545 



family without referring to the curiously-ridged quadrangular teeth from the 

 Chalk described under the name of Ptychodus, which appear to indicate an extinct 

 type of eagle-ray. In these teeth the highly-polished crown is ornamented with 

 large transverse or radiating ridges, surrounded by a more finely-marked marginal 

 area of variable width. They are arranged in longitudinal rows: the upper jaw 

 having the teeth of the middle row the largest, and those of the lateral rows 

 gradually decreasing in size ; while in the lower jaw the middle teeth are rather 

 small, and the two adjacent rows the largest. 



The Sting-Rays, — Family Trygonid^e. 



Apparently the most specialised members of the entire group are the sting- 

 rays, in which the pectoral fins are continued uninterruptedly round the extremity 

 of the muzzle, so that the whole of the margin of the very wide disc is formed by 

 these tins, in the centre of which is the more elevated head and body. The long 

 and slender tail, which is frequently armed with a serrated spine, is sharply 

 defined from the body ; and the median fins, if present at all, are either imperfectly 

 developed, or are modified into serrated spines. The forms with armed tails, to 

 which the name of sting-ray is alone strictly applicable, inflict very severe wounds, 

 dangerous not merely from the actual lesion, but apparently also from the pre- 

 sence of some poisonous substance. In the larger kinds these formidable spines 

 may be as much as 8 or 9 inches in length; and, as they wear out, they are from 

 time to time shed and replaced by new ones growing from behind. Very 

 numerous in species, and arranged under several genera, the sting - rays are 

 most abundant in the seas of the tropics, although some range into temperate 

 waters. 



The typical genus includes some twemVy-five species, one of which (Trygon 

 jKistinaca) ranges from the south of England westwards to America and east- 

 wards to Japan. In this group the greatly elongated and tapering tail is armed 

 with a barbed arrow-shaped spine, while the skin is either smooth or dotted over 

 with tubercles, the nasal valves unite to form a quadrangular flap, and the teeth 

 are flattened. Mainly characteristic of tropical latitudes, these rays are most 

 abundant in the Indian and Atlantic Oceans, although some species are inhabitants 

 of fresh-water lakes in Eastern Tropical America, The rough ray (Urogymnns 

 tisperriTrms), of the Red Sea and Indian Ocean, which may measure from 4 to 5 

 feet in length from the head to the root of the tail, is the sole representative of 

 a second genus, characterised by the long tail being devoid of either fin or spine, 

 although sometimes furnished with a narrow fold of skin below. The whole 

 of the body is thickly covered with teeth-like tubercles, the teeth themselves 

 being flattened. The third genus (Urolophus) — in which the tail is of medium length, 

 furnished with a distinct terminal rayed tin, armed with a, barbed spine, and some- 

 times with a rudimental dorsal tin. while the teeth are flattened — contains several 

 rather small-sized species from the tropical seas, and likewise an extinct one from 

 the Eocene rocks of Italy. A fourth genus (Pteroplatea), of which there are some 

 half-dozen representatives from temperate and tropical seas, is characterised by 

 the great width of the disc, which is at least twice as long as wide, and also 



vol. v. — 35 



