TRUE RAYS. 
54i 
deeply undulated, so that the prominences of one jaw tit 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 Raiibas. 
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 
tins is variable, and the rayed portion of the pectoral tins 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 case 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 lbs. 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 
