CHONDEOPTERYGII — PLAGIOSTOMATA. 313 



mainder of their organisation as the Teleostei. The Palceich- 

 thyes stand to the Teleostei in the same relation as the 

 Marsupials to the Placentaha. Geologically, as a sub-class, 

 they were the predecessors of Teleosteous fishes ; and it is a 

 remarkable fact that all those modifications which show an 

 approach of the ichthyic type to the Batrachians are found 

 in this sub-class. We divide it into two orders: Ghondrop- 

 terygii and Ganoidci. 



FIRST ORDER: CHONDROPTERYGII. 



Skeleton cartilaginous. Body with medial and paired fins, 

 the hinder pair abdominal. Vertebral column generally hetero- 

 cerccd, the upper lobe of the caudal fin produced. Gills attached 

 to the skin by the outer margin, with several intervening gill- 

 openings : rarely one external gill-opening only. No gill-cover. 

 No air-bladder. Two, three, or more series of valves in the 

 eonus arteriosus. Ova large and few in number,^ impregnated 

 and, in some species, developed within a uterine cavity. Embryo 

 with deciduous external gills.^ Males with intromittent organs 

 attached to the ventral fins? 



This order, for which, also, the name Masmobranchii has 

 been proposed (by Bonaparte), comprises the Sharks and 

 Eays and Chimaeras, and is divided into two sub-orders : 

 Plagiostomata and Holocephald. 



FIRST SUB-ORDER : PLAGIOSTOMATA. 



From five to seven gill-openings. Skull with a suspensorium 

 and the palatal apparatus detached. Teeth numerous. 



The Plagiostomes differ greatly among each other with 

 regard to the general form of their body : in the Sharks or 

 Selachoidei the body is elongate, more or less cylindrical, 

 gradually passing into the tail ; their gill-openings are lateral. 

 In the Eays, or Batoidei, the gill-openings are always placed 



1 See p. 167, Figs. 79, 81. ^ gee p. 136, Fig. 58. « See p. 167, Fig. 78. 



