On the Fossil Fishes of the Carboniferous Limestone Sciies of Great Britain. 411 
frequently the only parts of the fish preserved, and which occur throughout nearly 
all the strata in which fossils are found, is rendered very great, from the fact that 
only in a very few instances can the determination be based on a comparison of the 
fossil species with the existing ones. The species found in the more recent forma- 
tions may be satisfactorily associated with living forms : but lower than the Tertiaries 
the geological affinities of the fossil fishes become more and more a matter of 
generalized speculation. The fishes of the secondary formations differ almost en- 
tirely from those now existing, whilst in the still earlier primary rocks, the characters 
exhibited by the fossils are only in rare cases comparable with living species, and in 
no instance is this so apparent as in the group of palates now under consideration. 
Professor Agassiz originally included in his genus Psammodus all the teeth of 
Cestraciontes, of which the surface is neither wrinkled or reticulated, without crest, 
or longitudinal or transverse ridges. The forms of the teeth included in this descrip- 
tion were very various. The genus comprehended all the species whose crown 
was formed, as in Cestracion, of small vertical tubes opening on the surface and 
extending downwards into the tooth in the form of medullary canals; thus defined 
it included :— 
Psammodus gibberulus. Psammodus contortus. 
P. subteres. P. Icngidens. 
P. linearis. P. reticulatus. 
P. rugosus. P. magnus. 
P. porosus. P. tenuis. 
The discovery of further specimens convinced M. Agassiz that several of the 
palates regarded as species of Psammodus were of generic value. Their occurrence 
in strata extending from the Stonefield slate and clays of Shotover through the 
Permian and Coal Measures series to the Mountain Limestone, supported the 
inference that during these long periods several types of Cestracions existed, which 
whilst closely related in their dentition were sufficiently distinct to have constituted 
separate genera. 
The following modifications of the genus Psammodus were therefore adopted ; 
without, however, being regarded either as final or more than approximately 
correct :— | 
Helodus (Psammodus) gibberulus and subteres, &c. 
Chomatodus ‘4 linearis, &e. 
Psammodus . rugosus, porosus. 
Cochliodus aS contortus. 
Strophodus . longidens, reticulatus, magnus, tenuis, &c. 
The genera Ceratodus, Ctenodus, Acrodus and Ptychodus were also associated with 
the above, being regarded as more nearly related to the Cestracionts than to the Rays. 
* « Poiss. Foss.,” Vol. IIL, p. 103. 
