SCIENTIFIC SURVEY. 5J 



The bulbus arteriosusf has almost always only two opposite semi- 

 lunar valves. The branchial apertures are represented by simple 

 fissures on each side. There are four pairs of true and well 

 developed branchial ai-ches, each of which generally supports free 

 branchiae ; an air bladder is generally present. The neutral fins 

 vary in position, and are sometimes absent. This class embraces 

 by far the largest proportion of existing fishes, and have been 

 divided by Prof G. into five "natural and easily distinguished 

 orders." The first order he calls Teleocephalt, or perfect heads. 

 Let us examine this order more in detail. 



Order Teleocephau, Gill. 



The endo-skeleton is almost always perfectly developed. Body 

 generally covered b}'' ctenoid or cycloid scales, branchiae pecti- 

 nated. The supramaxillaries and intermaxillaries, are always 

 present and separate from each other. The sub-opercular bone is 

 almost invariably present. Many of the rays are articulated and 

 branched. Nearly all the fishes most esteemed as food, belong to 

 this order ; it is divisible into several sub-orders. Let us next 

 examine these. 



Suborder Physoclisti, Bona. 



It has been found that a large number of fishes have an air 

 bladder that is closed, and has no visible duct or communication 

 out of it. Such of the fishes of the order Teleocepha,li are put 

 in this sub-order which, from this fact, is called Physochsli, from 

 the Greek phusa, bladder, and kleisos, closed. The scales on 

 fishes of this sub-order, when present, are either ctenoid or 

 cycloid ; there are rarely osseous plates. The anterior rays of the 

 dorsal and anal fins, and the first ray of the ventrals are simple, 

 or spinous. The ventrals are generally more or less anterior. 

 The lower pharyngeal bones are small and triangular, sometimes 

 united, but genCTally distiact, the teeth are implanted on the 

 plain surface. 



Sub-Order Heterosomata, Bona. 

 Next, on further examination, we find some fishes that have 



t Fisties ai'e destitute of a double heart, such as the mammalia have. In some 

 it appears to be merely an expansion of the aorta provided with one or two valves 

 within. 



