921) DIVISION I. VERTEBRAL ANIMALS.—CLASS IV. PISCES. 
which sometimes are of large size, but oftener are contracted to small points, 
as scen on the skin of the Shark, and in the prickly tubercles of the Rays. 
The name /7acoidean is derived from the Greek word plax — broad plate. 
The fishes of the third order have the scales composed of horny matter, 
their posterior edges, i. e., the edges directed towards the tail, furnished 
with projections like the teeth of a comb. The order derives its name from 
this circumstance, the Greek Atecs (hLtenos, gen.), a comb, suggesting the 
designation Ctenotdean. The Perch represents this order. 
The fourth order (the Cyclotdeans) derives its name from the Greek /a- 
kilos —a circle. The Carp, Herring, and Salmon, whose scales have a 
rounded furm, with smooth, simple edges, are examples which all can easily 
examine. 
In regard to the above arrangement, Mr. Mudie well remarks, that, in 
comparing it “with that of Cuvier, we shall find that the Cyclodd fishes of 
Agassiz are, for the most part, the Malacopterygii of Cuvier; and that the 
Clenoid fishes of the former are generally the Acanthopterygtt of the latter. 
Further, the Placotd fishes of Agassiz correspond with the principal section 
of the Curtilaginous fishes of Cuvier, the Sturgeons and Chimere being 
alone excepted. The existing G'anoid fishes of Agassiz, however, were 
distributed by Cuvier amongst several different families. 
“The application of this method of arrangement to the various forms of 
extinet fishes, which geological research has brought to light, has given some 
extremely curious results. In the first place, it may be stated as a general 
fact, that of the Cycloid and Ctenoid orders, there are no remains what- 
ever in any formation anterior to the chalk, and that, consequently, the whole 
assemblage of existing fishes included in those two orders, probably about 
four fifths of those now living, had apparently no representative whatever in 
the more ancient seas. Even in the chalk, there seems to have been only 
two or three of the largest of the existing families — such as the Herring and 
Salmon Tribes, the Mackerel Tribe, and the Perch Tribe, which attained 
any considerable importance. The others are cither but slightly represented 
at that epoch, and have subsequently increased very considerably — such as 
the Eels and the Pleuwronectide ; or first came in during the Tertiary period 
—such as the Carps and the Mullets; or present themselves, for the first 
time, in our own epoch, which is the case (strange to say) with the large 
and important Cod Tribe. Further, no family belonging to these orders 
has disappeared from the ocean subsequently to its first introduction ; nor is 
there any that seems to have undergone any diminution. The other two - 
orders, although they now form so small a part of the inhabitants of our 
seas, were once the sole yertebrated tenants of the globe.” 
