174 
NORTH GALLERY. 
[UPPER 
rhomboidal, in a few it is round ; but the pattern of the external mark- 
ings varies in almost every species. 
Ganoid Fishes range from the newest Silurian strata upwards ; are 
most abundant in the lower Oolitic formations, diminish in the cre- 
taceous beds, and are reduced to very few genera existing at the present 
time. 
The order is commenced in Wall Case No. 1, by the Cephalaspides, 
a family peculiar to the Devonian period. The species of which it is 
composed were fishes in which the body was protected by large bony 
plates, was convex above, and flat beneath ; the pectoral fins were 
represented by large bony appendages, situated close behind the head ; 
and the tail was tolerably long, tapering, and furnished with small 
scales. The second family of the order, the Ccelacanthi, so called 
from the spines of their fins being hollow, occupy the compartments 
3 to 5 of the same Wall Case. In the 6th compartment, and lower 
division of the 7th, are arranged the Dipterines, including fishes of 
the Old Red Sandstone and Coal formations, having the body pro- 
tected by rhomboidal scales, and provided with two back fins, as well 
as two anal fins. The specimens exhibited are chiefly from Scot- 
land. In the 8th compartment are arranged the Acanthodians. The 
fishes of this family are characterized by the minuteness of their scales 
and having each fin armed with a strong bony spine. They are 
mostly from the Devonian of Scotland. 
The family of the Sauroids (see compartments 8 to 13), contains 
fishes which exhibit both the uneven-lobed and the even-lobed structure 
of tail, and is divided accordingly into two minor groups : the Sauroids 
have conical teeth mixed with minute prickly teeth. The scales vary 
considerably in form. This family presents living examples in the Bony 
Pikes (Lepidosteus) of the rivers and lakes of North America, and in 
the Bichirs (Polypterus) of the Nile. The Sauroid family is represented 
by numerous extinct species ranging through nearly all geological 
strata down to the Devonian. 
The Ganoid family called " Lepidoids " (see compartments 14 to 20) 
have the same rhomboidal scales as the Bony Pikes, but in general 
form they are shorter and have a greater vertical diameter; the teeth 
are of one kind only, and of a more or less conical form. 
The last family of the fishes with ganoid scales is the Pycnodonts, - 
of which all the principal genera will be found in compartments 21 
and 22. Like the members of the preceding group, these fishes have 
a short, high, and compressed form. Their teeth are usually large, 
rounded, and with low crowns admirably fitted for crushing shell-fish. 
Many of the species are from the Jurassic (or Oolitic) rocks ; some are 
from the Chalk, and a few extend into the Tertiary formations, but 
there are no known living species. 
In the preceding two great divisions of Fishes many of the families 
of which they are composed are extinct, and a very large proportion of 
the species is confined to the geological strata beneath the Chalk, 
whilst those which now come under consideration belong entirely to 
the Chalk and Tertiary formations, and the families have living re- 
