DAVIS : FOSSIL FISH EEMAINS. 
53 
one before each dorsal fin, but there is no evidence to shew 
whether the fossil Orodonts had such spines ; '^the discoveries of 
further examples may serve at some future time to shew whether 
they had or not : so far as the evidence of the specimens found 
in Yorkshire g-oes, it would seem to indicate that they were not so 
defended. 
Several species of the genus Lophodus have been found in 
the upper beds of limestone in Wensleydale. The genus was 
originated by M. Rowanowsky, in 18G4 (" Bull, de la Soc. Imperi- 
al des Naturalists de Moscow," p. 160), and embraces the teeth 
of fishes which are closely allied with Orodus but present several 
features which had hitherto been considered to be characteristic 
of Helodus. Lophodus comprises teeth in which the crown of the 
tooth is more or less conical in outline with a corresponding con- 
cavity of the base, whilst in the teeth of Helodus there is no such 
concavity, and the crown, somewhat expanded laterally over the 
base, rises up from all sides to the apex. The base of the crown 
in Lophodus is contracted on each side and curved inwards to 
the root. 
The Cochhodontid^ are well represented in the Yorkshire lime- 
stones, so far as number of genera goes, but the number of speci- 
mens found have not been very great. The genera comprise 
species in Cochliodus, Deltodus, Deltoptychius, Psephodus and 
Poecilodus ; they differ very considerably from each in many 
minor peculiarities, but they agree in others of greater impor- 
tance. The distinguishing characteristic of the group is, that the 
teeth "^grow or increase in size, not as in most fishes by the old 
and worn teeth being replaced from behind or below, but by 
continuous or repeated additions to the inner or posterior margin 
of the surface of the tooth, so that the same tooth is always 
increasing in size with the growth of the fish, and assumes an 
inrolled or convoluted form. An analogue of this pecular growth 
is seen in that of the testaceous covering of the mollusca, which 
though it assumes a wonderful variety of forms, increases in size 
