348 nOLOCEPHALA. 



Subclass IV. CHONDROPTERYGII. 



Skeleton cartilaginous ; skull without sutures. Body 

 Avitli medial and paired fins, the hinder pair abdo- 

 minal; caudal fin Avith produced upper lobe. Gills 

 attached to the skin by the outer margin, with several 

 intervening gill-openings ; rarely one gill-opening only. 

 No gill-cover. No air-bladder. Three series of valves 

 in the bulbus arteriosus. Intestine with a spiral valve. 

 Optic nerves commissurally united, not decussating. 

 Ovaries with few and large ova, which are impreg- 

 nated and, in some, developed internally. Embryo 

 with deciduous external gills. Males with prehensile 

 organs attached to the ventral fins. 



This subclass, for which also the name Elasmobranchii has boon 

 proposed {Bonaparte), comprises the Sharks and Hays, and is di- 

 vided into two orders :— 



I. Holocephala. One external gill-opening only (p. 348). 



II. Plagiostomata. From five to seven gill-openings (p. 353). 



Order I. HOLOCEPHALA. 



One external gill-opening only, covered by a fold of 

 the skin, which encloses a rudimentary cartilaginous 

 gill-cover ; four branchial clefts within the gill-cavity. 

 The maxillary and palatal apparatus coalescent with 

 the skull. 



Holocephala, 3IiUlcr, Abhandl. Ak. Wiss. Berl. 1834, p, 74. 

 Uuc family only. 



