ORDERS OP FISHES. 223 



that is to ^y, of detached grains, tubercles, or plates. There 

 are two pairs of fins, corresponding to the fore and hind limbs, 

 and the ventral fins are placed far back, close to the anus. 

 The heart consists of an aiu-icle and ventricle ; and the bulbus 

 arteriosus is rhythmically contractile, is provided with a dis- 

 tinct coat of muscular fibres, and is furnished with several 

 transverse rows of valves. The gills are fixed, and form a 

 number of pouches, which open internally into the pharynx, 

 and communicate with the outer world by a series of aper- 

 tures placed on the side of the neck (Fig. 109). The intestine 

 is very short, but, to compensate for this, the mucous mem- 

 brane is thrown into a fold, which winds round the intestine 

 in close turns from the pyloric orifice of the stomach to the 

 anus, and which thus greatly increases its absorbing surface. 



The best-known members of this order are the Sharks and 

 Rays, but numerous extinct forms testify to its great abun- 

 dance in past geological epochs. 



Tabular Tiew of the Divisions op the Elasmobranchii. 



SuB-OKDEE I. HoLOCEPHALi. — The mouth placed at the end of the head, 

 and the external opening of the gills in the form of a single gill-slit. 



The best-known member of this sub-order is the ChimcBra monstrosa, 

 sometimes called the "King of the Herrings." 



SuB-OKDEK II. Plagiostomi. — Mouth transverse, placed on the under sur- 

 face of the head ; external opening of the gills in the form of several slits on 

 each side of the neck, not protected by a gill-cover. 



Jfam. a. Cesiraphori. — Ex. Port-Jackson Shark. 



Fam. b. Selachii. — JElx. Sharks and Dog-fishes. 



I^am. c. Batides. — Ex. Rays. 



Oedeb VI. Dipnoi (Gr. di, double; pnoe, breath). — This 

 order is a very small one, and includes only the very singular 

 Mud-fishes {Lepidosiren)* which are of great interest from 

 the many points of affinity which they exhibit to the Am- 

 phibia. The body of the Mud-fish (Fig. 110) is completely 

 fish-like, and is protected by a covering of small, horny, over- 

 lapping scales, which have the cycloid characters. There are 

 two pairs of limbs, but these are in the form of awl-shaped 

 organs, each supported by a single jointed cartilaginous rod. 

 The pectoral hmbs have a membranous fringe inferiorly, and 

 the ventrals are placed very far back. There is also a median 



* Eeccntly a singnlar flsh has been discovered in the rivers of Queensland (Anstralia) 

 which will probably have to bo referred to the order Dipnoi; but our knowledee about it is 

 still imperfect. 



