CHIMERAS. 99 



Eastern North America. A fine example from the Doggerbank 

 is placed outside the case. Scaphirhynchus is a closely allied genus, 

 of which four species are known — one from the river-system of the 

 Mississippi, and the three others from Central Asia. 



The Polyodontida, or Sword-bill Sturgeons, which have the 

 snout produced into an exceedingly long shovel-like or conical 

 process, contain two species — one from the Mississippi, Polyodon 

 folium, growing to a length of about six feet ; the other, Psephurus 

 gladius (exhibited in a tank opposite wall-case 29), inhabits 

 the large rivers of China, the Yantsekiang and Hoangho. The 

 great depth of the rivers in which these fishes live, as well as the 

 turbid condition of their water, renders the organ of sight almost 

 useless : the eyes of these Sturgeons, therefore, are remarkably 

 small; and to obtain their food they evidently use the rostral 

 process in stirring up the mud at the bottom, thus dislodging 

 and finding the small animals on which they prey. 



Order VIII. CHONDROPTERYGII. 



The skeleton is cartilaginous, with the vertebral column generally 

 heterocercal. The body has median and paired fins, the hinder 

 pair being abdominal. The gills are attached to the skin by the 

 outer margin, with several intervening gill-openings (except in the 

 Chimasras, which have only one gill-opening on each side) ; a gill- 

 cover is absent, as also the air-bladder. Some are viviparous, 

 but the majority are oviparous. A pair of semiossified appendages 

 of the pubic, called claspers, are characteristic of all male indivi- 

 duals. These appendages are sometimes armed with hook-like 

 osseous excrescences. They are irregularly longitudinally convo- 

 luted, and when closely adpressed to each other form a canal open 

 at their extremity. The ova are large and few in number, and 

 invested with a tough leathery envelope or shell (fig. 90), presenting 

 great variety of shape. 



This Order comprises the Chiniseras, Sharks and Rays, and is 

 divided into two Suborders — Holocephala and Plagiostomata. 



Holocephala, or Chimseras (Case 30). 

 These are chiefly characterized by having one external gill-opening 

 only, covered by a fold of the skin ; also their dentition strongly 



