CHONDROPTERYGII — PLAGIOSTOMATA. 313 



mainder of their organisation as the Teleostei. The Palmich- 

 thyes stand to the Teleostei in the same relation as the 

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

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

 remarkable fact that all tliose modifications whicli show an 

 approach of the ichthyic type to the Batrachians are found 

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

 terygii and Ganoidei. 



FIRST ORDER : CHONDROPTERYGII. 



Skeleton cartilaginous. Body witJi medial and paired fins, 

 the hinder pair abdominal. Vertebral eolumn generally hetero- 

 cercal, the upper lobe of the eaudal fin produced. Gills attached 

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

 openings : rarely one external gill-opening only. Ho gill-cover. 

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

 conus arteriosus. Ova large and feu) in number} impregnated 

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

 ivith deciduous external gills.^ Males ivith intromittent organs 

 attached to the ventral fins? 



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

 been proposed (by Bonaparte), comprises the Sharks and 

 Pcays and Chimseras, and is divided into two sub-orders : 

 Plagiostomata and Holocephala. 



FIRST SUB-ORDER : PLAGIOSTOMATA. 



From five to seven gill-opienings. Skidl ivith 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 

 Selaehoidei the body is elongate, more or less cylindrical, 

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

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



1 See p. 167, Figs. 79, 81. ^ ggp p, 135^ Yig. 58. => See p. 167, Fig. 78. 



