124 



CHAPTER X X X L 



ORDERS OF FISHES. 



Order I. Teleostei. — This order includes the great majority 

 of fishes in which there is a well-ossified endoskeleton, and 

 it corresponds very nearly with Cuvier's division of the 

 " osseous " fishes. The Teleostei are defined as follows : 

 The skeleton is usually well ossified ; the cranium is provided 

 with cra7iial hones, and a mandible is present ; whilst the 

 vertebral column almost always consists of more or less com- 

 2Jletely ossified vertehrm. The pectoral arch has a clavicle ; and 

 the tivo jMirs of limbs, when p)'t'^sent, are in the form of fins 

 supported by rays. The gills are free, pectinated or tufted in 

 shape, a bony gill-cover and branchiostegal rays being always 

 developed. The branchial artery has its base developed into a 

 bidbus arteriosiis ; but this is 7iever rhythmically contractile, 

 and is separated from the ventricle by no more than a single 

 row of valves. 



The scales in the Teleostean fishes are generally thin, 

 horny, flexible plates, which overlap one another, and have 

 the " cycloid " or " ctenoid " characters. The order, therefore, 

 corresponds in a general way with the orders Ctenoidei and 

 Cycloidei of Agassiz. Some of the Teleostean fishes, how- 

 ever, are provided with ganoid scales. 



Excluding the Leptolepidcc, which are sometimes referred 

 to this order, the Teleostei do not seem to have any repre- 

 sentatives in times anterior to the Cretaceous period — that 

 is, towards the close of the Mesozoic period. From this 

 time on, liowever. Bony fishes with cycloid or ctenoid 



