58 
DAVIS : FOSSIL FISH EEMAINS. 
the Yorkshire Limestone is comprised in the Petalodontidae. The 
g-roup was instituted by Messrs. Newberry and Worthen (Geology 
and Palaeontology of Illinois, vol. ii, p. 31 ), and embraces several 
genera of fishes known only by their teeth, and so far as at present 
discovered, confined to the Carboniferous series of rocks. The 
genera which occur in this county are Petalodus, Agass ; Petalop- 
sodus, Davis; Polyrhizodus, McCoy; Ctenopetalus, Agass; and 
Petalorhynchus, Agass. 
The genus Petalodus is represented by the well-known species 
Hastingsi83 and acuminatus. The latter is fairly abundant and 
frequently of large size, but otherwise the genus does not offer 
any features of peculiar interest. Polyrhizodus is represented by 
only one small and very rare species. This genus frequently 
occurs in the Limestone of L-eland, and appears to be of sufficiently 
diversified and well defined character to necessitate the formation 
of at least six species. The teeth are in many respects similar to 
those of Petalodus. The strong, well-developed crown with a 
sharp cutting-edge is equally characteristic of the two genera ; the 
principal difference consists in the formation of the basal portion 
which was inserted in the jaw of the fish ; in Petalodus the base 
is composed of a single massive root, whilst in Polyrhizodus, as 
implied in the name, the root is divided into a number of radicals 
or rootlets, varying in number from four or five, to sixteen or 
twenty. This character of the root of Polyrhizodus is very pi^cu- 
liar, and does not occur in any other group of fishes, either recent 
or fossil, The teeth of most of the Plagiostomous fishes are 
simply attached to the jaw by their under surface in a somewhat 
loose manner, and as might be supposed, not being firmly implant- 
ed in the jaw, but only 07i it, they are easily displaced. To 
compensate for this liability to loss, the fishes are provided with 
several rows or series of teeth, which until required, are arranged 
on the inner side of the jaw, their sharp edges pointing towards 
the palate. On the displacement or breakage of the teeth in 
active use, they are replaced by others from the reserve ; this 
