FOSSIL RAYS. 291 
cates the number of extinct genera and species 
of the family of Sharks, that occupied the waters 
throughout these early periods of time. Not less 
varied are the forms of palate bones and teeth, 
in the same formations that contain these 
spines ; but as the cartilaginous skeletons to 
which they belonged have usually perished, 
and the teeth and spines are generally dis- 
persed, it is chiefly by the aid of anatomical 
analogies, or from occasional juxtaposition in 
the same stratum, that their respective species 
can be ascertained. 
Fossil Rays, 
The Rays form the fourth family in the order 
Placoidians. Genera of this family abound 
among living fishes ; but they have not been 
found fossil in any stratum older than the Lias ; 
they occur also in the Jurassic limestone. 
Throughout the tertiary formation they are 
very abundant ; of one genus, Myliobates, there 
are seven known species ; from these have been 
derived the palates that are so frequent in the 
London clay and crag. (See PL 27"^, B. Fig. 
14.) The genus Trygon, and Torpedo, occur 
also in the Tertiary formations. 
attached to strong muscles ; but articulate with a bone beneath 
them. The Spine of Balistes also is kept erect by a second 
spine behind its base, acting like a bolt or wedge, which is 
simultaneously inserted, or withdrawn, by the same muscular 
motion that raises or depresses the spine. 
