THORNBACK RAT. 
101 
sometimes the skin is covered with evident roughness; aciicum- 
stance which has led some ohservers to suppose such examples 
to constitute separate species. I have also met with some 
singular varieties, and of an opposite kind, which require notice 
in this place. A male example, taken in comparison with 
another of the same species, agreed with it in every particular, 
except that its skin was altogether smooth, except a few rough 
grains on the point of the snout, and the usual spines over 
the eyes and on the border of the disk at the pectoral fins. 
The usual roughness of the skin was altogether absent, and not 
a single hooked spine existed otherwise on the body. The 
colour was pale dusky brown, and not mottled, as is constantly 
the case in ordinary examples; but having some fine longitudinal 
lines. The liver was remarkably small, and black as if it had 
been soaked in ink. 
In another example, which in form also closely resembled the 
common Thornback Ray, no hook or spine could be discovered, 
except a row that passed from the head along the back and 
down the middle of the tail; with none behind the eyes or on 
the sides of the tail. Surface of the disk very rough, more 
BO than in the common examples of this Ray; the teeth flat as 
in that fish, and in the centre of each tooth was a depression, 
which was softer than at the border; so that each separate 
tooth resembled a wide ring. The surface of the disk was an 
intense black, but near the head bordered with a lighter colour; 
and over the surface were scattered a few round spots of a 
still darker colour. 
1 have not thought it necessary to regard these examples as 
forming distinct species; principally because there are none 
described by other ohservers, to which they can he referred; 
the It. ruhus of Linnseus and other authors appearing to me to 
be a very uncertain species; and the resemblance of form was 
in all respects, except the particulars here given, like that of 
the Thornback Ray. 
