FISHES. 57 



which articulates with the corresponding half of the lower jaw. 

 Both upper and lower jaws are armed with teeth. 



The hyoid consists of a pair of long and strong lateral pieces 

 and a single mesial piece. From the former cartilaginous filaments 

 (representing branchiostegals) pass directly outwards. Branchial 

 arches (br), similar to the hyoid, succeed it; and are suspended 

 from the side of the foremost part of the spinal column, and, like 

 the hyoid, bear a number of cartilaginous filaments. 



The vertebral column is composed of a series of centra (c), coni- 

 cally excavated in front and behind, with a central canal through 

 which the notochord is continued, and with neural (na) and 

 hsernal (ha) apophyses. The caudal extremity of the vertebral 

 column shows a heterocercal condition, i.e. its axis is turned upwards 

 and the hsemapophyses are much more developed than the neurapo- 

 physes. The vertical fins are supported by interneural and inter- 

 hsemal cartilages, to which the fin-rays are attached without articu- 

 lation. 



The scapular arch is formed by a single coracoid cartilage (co) 

 bent from the dorsal region downwards and forwards, not suspended 

 from the skull as in the majority of Teleosteans. Behind, at the 

 point of its greatest curvature, three carpal cartilages are joined to 

 the coracoid, which are distinguished as propterygium, meso- 

 pterygium, and metapterygium, the former occupying the front, the 

 latter the hind margin of the fin. Several transverse series of 

 styliform cartilages (ph) follow; they represent the phalanges, to 

 which the horny filaments (r), which are imbedded in the skin of 

 the fin, are attached. 



The pubic is represented by a single median transverse cartilage 



(pu), with which a tarsal cartilage articulates. The latter supports 



the phalanges and fin-rays. To the end of this cartilage is also 



attached, in the male, a peculiar accessory generative organ or 



clasper. 



The Class of Fishes is divided into 3 Subclasses and 9 Orders : — 



Subclass I. Teleostei. Heart with a non -contractile bulbus 

 arteriosus ; intestine without spiral valve ; optic nerves decussating. 

 Skeleton ossified, with completely separated vertebroe. 



Orders : — 1. Acanthopterygii. 2. Acanthopterygii Pharyngo- 



