54 
DAVIS : FOSSIL FISH KEMAINS. 
by the addition of repeated layers to the open margin of the 
shell."* The genera Cochliodus, Deltodus and Deltoptychius 
exhibit a distinctly inrolled appearance in a transverse section, 
whilst Psephodus and Poecilodus are flatter and increase in size by 
the radial expansion of the triturating- surface. Hitherto, the 
complete dentition of any member of the group has not been 
found, but specimens of several of the genera shew that there 
were three teeth on each jaw ; those in front which approached 
and joined at the symphysis of the jaw were the smallest, and the 
second and third pairs increased in size backwards. Those situat- 
ed most posteriorly are expanded so as to form a wide, more or less 
convex surface, admirably adapted for crushing and triturating 
vegetable substances for food, which in all probability formed the 
principal part of their sustenance. The teeth were attached and 
partially imbedded in a strong cartilaginous jaw, which possibly 
extend inwards as to form a palate to which other teeth may have 
been attached. That teeth similar to those hitherto nanjed Helo • 
dus may have occupied the central portion of the palate, is 
rendered probable by the discovery in some of the American 
strata of specimens of Cochliodus and Helodus in close apparent 
relationship, though the specimens fouod in this country have not 
indicated such a combination. 
As already stated, the teeth of Cochliodus have a distinctly 
inrolled or helicine configuration whilst those of Psephodus are 
much flatter and very slightly curved. The latter vary greatly 
in size, as well as to a smaller extent, in form. The teeth of 
Cochliodus have been found not only connected together but the 
teeth of the two rami of the jaws united by their cartilaginous 
supports ; the teeth of the Psephodus, however, have not been 
discovered so united, but their arrangement has been proved to 
be similar to those of Cochliodus by the peculiarly conca^^e under 
surface. If one of the largest teeth be examined it will be found 
that the widest posterior edge is rounded, whilst the opposite edge 
* Pioc. Roy. Soc, Dublin, New Ser. Vol. I., p. 415. 
