SELACHII. 161 



also, the embryos of the Plagiostomes exhibit notable peculiarities, 

 especially in the possession of external branchial filaments (fig. 605), 

 which are lost long before birth. 



Almost all the Plagiostomes are marine ; only a few of them are 

 found in the larger rivers of America and India. They are all 

 carnivorous, and feed on large fishes, or Crustacea and mollusca. Some 

 few (Torpedo) possess an electric organ. 



With the exception of Pleur acanthus, remains of spines and teeth 

 only are preserved in the Palaeozoic formations. From the secondary 

 period onwards the remains are more complete and numerous. 



Sub-order 1. Holocephali. 



Selachians with maxillo-palatine apparatus firmly fused to the skull,, 

 with single, external gill slit on each side and small opercular 

 membrane. 



The thick strangely formed head is provided with large eyes which 

 Sp 



M 



FIG. 605. Embryo of AcantJiias with external gills. Sp, spiracle ; H, mouth; Nb, stalk of 



yolk-sac. 



are without lids. The mouth is small and lies on the under surface 

 of the snout. The maxillo-palatine (palato-quadrate) bar is firmly 

 fused with the skull, while the lower jaw articulates with a styliform 

 process of the skull (hyomandibular). The jaws have but few 

 teeth (four above, two below). The naked skin is traversed by the 

 large passages of the lateral sense organs. There are no spiracles. 

 The vertebral bodies are replaced by thin calcareous annular incrusta- 

 tions in the sheath of the notochord. They lay eggs with horny 

 shells. 



Fam. Chimaeridae (Sea-cats). Chimcera momtrosa L. (fig. 606), Northern 

 Seas and Mediterranean ; Callorhynchus antarcticus Lac., Cape and Pacific. 



Sub-order 2. Plagiostomi. 



Selachians ivith wide, transverse mouth, which is placed far back, 

 separate vertebral bodies, and a more or less reduced notochord. There 

 are five (exceptionally six or seven) external gill slits on each side. 



The nasal apertures are placed on the under surface of the snout. 

 VOL.. ii. 



