6 



by differences observed in the vascular and nervous systems. Four 

 at least of these are very distinct, and Agassiz* has recently even sug- 

 gested that they are entitled to rank as classes, basing his opinion chiefly 

 on the difference of development in each group. 



Representatives of five of the subclasses of M idler are found on the 

 Eastern shores of North America. In accordance with the suggestion of 

 Prof. Baird, a synopsis is given of the most obvious and important char- 

 acters of each. 



Subclass TELEOSTEI Miiller. 



The first subclass has been named Teleostei by Miiller. The endo- 

 skeleton is almost always osseous. The scapular arch is suspended from 

 the skull ; the supra scapula generally connected with the mastoid and 

 paroccipital bones. The exo-skeleton is generally in the form of cycloid 

 or ctenoid scales, but sometimes the body is naked and sometimes 

 covered with bony scales, plates or spines. The optic nerves cross each 

 other in their .passage from their respective lobes to the eyes. The bulbus 

 arteriosus has almost always only two opposite semilunar valves. The 

 branchial apertures are represented by simple fissures on each side. 

 There are four pairs of true and well developed branchial arches, each of 

 which generally supports free branchiae. An air-bladder is generally pre- 

 sent. The ventral fins vary in position and are sometimes absent. 



This subclass embraces by far the largest proportion of existing fishes. 

 If we consider the Plectognaths, the Lophobranchiates and the Siluroids, 

 as members of the group, we may divide it into five natural and easily 

 distinguished orders. Agassiz considers the three orders above mentioned 

 as perhaps more nearly allied to the Ganoids. For the present, we prefer 

 to retain them among the Teleostei. 



The orders may be thus characterized : 



Order TELEOCEPHALI Gill. 



The endo-skeleton is almost always perfectly developed. The body is 

 n-enerally covered by ctenoid or cycloid scales. The branchiae are pecti- 

 nated. The supramaxillaries and intermaxillaries are always present and 

 separated from each other.f The subopercular bone is almost invariably 

 present. J Many of the rays are articulated and branched. 



This order embraces the largest number of recent fishes, and is cosmo- 

 politan in distribution. Almost all of the fishes most esteemed as food 

 belong to it. 



It is divisible into several suborders. 



to 



* ' ' Contributions to the Natural History of the United States of America, by- 

 Louis Agassiz." Boston, 1857, vol. i. 



t In the family of Mormyroids, the intermaxillary bones are united, but 

 they are distinct from the supramaxillaries. 



% It is absent in the Notopteroids. 



