70 NATURAL HISTORY. (Fossils.) [NORTH 
shire; the other, the lake district of Cumberland, Westmoreland, and 
Lancashire. 
Room II. 
In this room are arranged the Fossil Fishes ; and, with regard to the 
classification adopted, it may be observed that it does not differ very 
materially from the system of M. Agassiz, as developed in the “ Re- 
cherches sur les Poissons Fossiles ,” yet modifications have been made, 
and in these modifications the views of Professors Muller and Owen 
have chiefly been consulted. The four great divisions of Placoids , 
Ganoids , Ctenoids, and Cycloids , established by M. Agassiz,—divisions 
founded upon differences observable in the form and structure of the 
scales,—if they are not all of them strictly natural, may still he referred 
to, the definitions applying, in a general way, to the groups as here 
arranged. 
The first division of the Fishes (the Placoids) comprises the Sharks 
and Rays, and allied groups. Having a more or less cartilaginous 
skeleton, this part is rarely preserved in a fossil state, hence our know¬ 
ledge of the extinct species is derived chiefly from such hard and 
durable parts as the teeth and spines,—small objects which are more 
conveniently arranged in horizontal cases, and will be found in the 
small tables under the windows. The first Table is entirely occupied 
by teeth of various species of sharks, found, for the most part, in the 
lower tertiary and chalk formations. The second Case contains— 
on one side, numerous vertebrae of the shark tribe, some of which in¬ 
dicate animals of very large dimensions; on the other side is a series of 
“ palates ” of the Ray and Skate tribe ; they consist of the masses of 
teeth with which the jaws were furnished, viz. a central row of teeth, 
in the form of flat, transverse, enamelled plates, wfith angular margins 
dovetailing with smaller, and equally flat-crowned teeth. The plates 
in question, being implanted on jaws in which the bone has but a small 
proportion of earthy matter (being chiefly cartilaginous), are most 
frequently found detached. Near the “ palates ” are placed some 
scales and barbed spines, belonging to the same tribe of fishes. 
The scales are composed of a rounded, hard disc (originally attached 
to the skin), from the centre of which springs a tooth-like prickle. 
Similar scales are seen in the Thornback, a common kind of skate 
found on the British coast.* The barbed spines were attached to 
the upper surface of the long slender tails of the “ Sting Rays ” 
( Myliobatis ). The visitor will see similar spines upon examining 
the recent fishes of this tribe which are arranged in the adjoin¬ 
ing gallery; and his attention may at the same time be directed to 
the species of Sharks placed in Room V. of the same gallery. The 
Port Jackson Shark, and several other sharks deposited in the same 
Case with it, have, it will be seen, two back fins, and each of these 
supported in front by a strong spine : the teeth of the fish mentioned, 
* The skin in the Plaeoid fishes is protected by hardened bodies which are ex¬ 
tremely variable in size and structure, but never assume the appearance of true 
scales, comparable to those of ordinary fishes. Commonly, they are of a star-like 
form, with a central projecting point; and often (as in the true Sharks) they are 
extremely minute and crowded together, and it is the skin of such fishes which 
forms the article of commerce termed shagreen. 
