DAVIS : FOSSIL FISH REMAINS. 
47 
siderable variety of plagiostomous fishes which have since disap- 
peared. Prof. Agassiz considered that a large proportion of the 
Carboniferous fishes bore a greater resemblance to the Cestracion,or 
Port Jackson Shark, than to any other existing genus ; the latter 
is represented by one or two existing species only, and appears 
from its comparatively small numbers and localized extent to be 
almost on the point of extinction. An important characteristic of 
the sharks of the older geological ages lies in their possessing, to 
a much larger extent than at present, large fin-spines, in many 
instances highly ornamented. 
The Ganoidei, which comprises an enormous assemblage and 
great variety of fishes in the ancient geological periods, is now 
reduced to a comparatively small number of species inhabiting the 
rivers of America and Africa, and the sturgeons which are marine. 
The typical ganoid, represented by the Garpike (Lepidosteus) of 
the American rivers, or the fossil Megalichthys, is covered with 
a coat of rhomboidal, thick, bony scales, with an enamelled sur- 
face. There are however, many exceptions to this character, and 
the living Amia of the American lakes, covered with thin imbri- 
cated scales, is devoid of enamel, they are of small size, very 
similar to some of the bony fishes. 
The groups of fossil fishes found in the Yorkshire Limestone 
Series included in the sub-order Plagiostomata are the following. 
Hybodontidfe, represented by Cladodus, Pristicladodus and 
Glyphanodus. 
Orodontidae Orodus, Lophodus and Diclitodus. 
Cochliodontidae Cochliodus, Deltodus, Psephodus, 
Deltoptychius and PcBcilodus, 
(perhaps Pleurodus.) 
Petalodontidae ,, Petalodus, Polyrhizodus, Petalop- 
sodus, Ctenopetalus, and Pet- 
■' alorhynchus . 
Psammodontidse ,, Psammodus. 
Copodontidae Dimyleus. 
Ichthyodorulites , , „ Cladacanthus and Physonemus. 
Besides these, the teeth of the genus Pristodus, which can- 
not be included with any of the above groups, but whose affinities 
