TRUE RAYS. 541 



deeply undulated, so that the prominences of one jaw fit into hollows in the 

 opposite one ; the individual teeth being more or less diamond-shaped. These rays 

 do not exceed 8 feet in length. Several extinct genera occur in the upper 

 Jurassic strata of Europe 



THE TRUE RAYS OR SKATES, Family 



Represented by upwards of seven British species, all of which belong to the 

 typical genus, the true rays are characterised by the broad and rhombic form of 

 the disc, the skin of which is generally marked with tooth-like rugosities. The 

 tail has a longitudinal fold on each side, the degree of development of the median 

 fins is variable, and the rayed portion of the pectoral fins extends to the muzzle. 

 With the exception of occasional traces in the tail, electric organs are wanting. 

 Of the typical genus we take as a 

 well-known example the common 

 British thornback (Raia clavata), 

 of which two examples are shown 

 in the illustration. In this genus 

 the tail is very sharply defined 

 from the disc, which is generally 

 covered with rugosities ; the 

 pectoral fins stop short of the 

 extremity of the muzzle; the 

 pelvics are deeply notched, with 

 a stout front cartilaginous ray; 

 the tail carries two dorsal fins, 

 and the caudal is rudimentary, or 

 wanting. Most of these skates are remarkable for presenting sexual differences, 

 which in the thornback and several other species display themselves in the dentition, 

 the teeth of the males being sharp and pointed, while those of the opposite sex 

 are blunt and flattened. Whereas the males of all the species are armed with patches 

 of claw-like spines lying in grooves on the upper surface of the pectoral fins, and 

 frequently also on the sides of the head, the females of some species have a kind 

 of buckler of asperities on the disc, which is wanting in the other sex. In other 

 cases the variation takes the form of a difference in colour. The numerous 

 members of this genus are in the main characteristic of the cooler seas, and while 

 they are more abundant in the Northern than in the Southern Hemisphere, some 

 of them approach nearer to the Arctic and Antarctic Circles than is the ca.se with 

 any other rays. The flesh of all of them is eatable, that of many species being 

 commonly sold as an article of food. The common skate (Raia batis), which is 

 ordinarily of from 2 to 4 feet in length, is greyish white in colour, with black 

 specks, the whole upper surface being more or less granulated. Buckland records 

 an unusually large specimen which weighed 90 Ibs. The thornback takes its name 

 from having the whole of the upper surface studded at intervals with the above- 

 mentioned claw-like spines ; the tail being also armed with longer spines, of which 

 a row runs along the middle of the back. The prevailing colour of the upper 



JAWS OF MALE (A) AND FEMALE (B) THORNBACK SKATE. 



