Classification of the Order Ostariophysi. 555 



Silnroids, is formed by a process of the supraoccipital and 

 by tlnee plates wlilch are expansions of the distal ends of 

 the " internenrals " (l)asalia + radalia) of the tirst three rays 

 of the dorsal tin. These bones are directed obliqnely up- 

 wards and backwards, in such a way that the fiu-ray articu- 

 lates with the distal end of its own radial and also rests 

 upon the upper part of the one behind it; when the second 

 dorsal ray forms a spine it is the third interneural which 

 becomes enlarj^ed to snpport it, whilst the second bears the 

 short anterior spine and the first is set free^ sometimes ex- 

 panding to form the tirst nuchal plate. 



The air-bladder, when well developed, differs from that of 

 the Cyprinoids in that it is divided into anterior and poste- 

 rior divisions not by a constriction but by an internal 

 partition ; the posterior division is rendered non-distensible 

 by the development of longitudinal and transverse septa, 

 whilst the anterior gives rise to the pneumatic duct, is 

 connected with the tripus, and usually extends laterally to 

 beneath the skin above the pectoral fin. The parapophysis 

 of the fourth vertebra is usually divided into an anterior 

 branch, decurved and firmly attached distally to the stem of 

 the supra-cieitbrum, and a posterior horizontal branch ; 

 these give attachment to the anterior and dorsal w alls of the 

 anterior chamber of the air-bladder. 



jNJodifications of the simple condition just described may 

 be of two kinds — (1) The anterior ramus of the para- 

 pophysis of the fourth vertebra loses its attachment to the 

 supra-cleithrum, its proximal end forms a thin laminar stem 

 and its distal end an expanded plate, inserted in the wall of 

 the air-bladder and furnished with a muscle attached ante- 

 riorly to the skull, the whole forming an '' elastic spring 

 mechanism." A posterior diverticulum of the air-bladder 

 is often found in Siluroids with this arrangement, which 

 occurs in the Doradidae, Pangasiidae, Synodontidse, and 

 Malopteruridae. 



(2) The air-bladder is reduced, the posterior part dis- 

 appearing and the anterior part becoming divided into two 

 lateral portions which may be completely disconnected ; 

 concurrently the fourth and fifth transverse processes tend to 

 surround and encapsule the air-bladder, in the most specialized 

 types forming complete bony cylinders, open only at their 

 outer ends. 



Giinther* regarded the Siluroids as a single family, and 

 his classification, based on external characters only, 



* Cat. Fish. v. (1864). 



37* 



