ANATOMY OF VERTEBRATES. 13 



the border free ; a single external gill-aperture on each side ; 

 opercular and branchiostegal rays. Oviparous ; ova few and large. 



Fam. 1. Chimceridce. Ex. Chimrera, Callorhynchus. 



2. Edaphodojitidce. Ex. Edaphodus, Ischiodus, Elasmodus. 



Order X. PLAGIOSTOMI. 



Endoskeleton cartilaginous or partially ossified ; vertebras 

 biconcave ; exoskeleton as placoid granules or tubercles, spiny in 

 some. Mouth transverse on the lower surface of the head. Optic 

 nerves commissurally united, not decussating. Valves of bulbus 

 arteriosus multiserial. Gills attached to the skin by the outer 

 margin, with intervening gill-apertures, five or more in number, 

 on each side ; no operculum. 



Suborder I. Cesteaphori. 1 

 (Spine in front of each dorsal fin ; back teeth obtuse.) 



Fam. 1. Hybodontidce. Ex. Hybodus. 



2. Cestraciontldce. Ex. Cestracion. 



Suborder II. Selachii. (Sharks, branchial apertures lateral.) 



Fam. 1. Notidanidce. Ex. Grey Shark. 



2. Spi?iacidce. Ex. Piked Dog-fish. 



3. Scylliadcs. Ex. Spotted Dog-fish. 



4. Nictitantes. Ex. Tope. 



5. Lamnidce. Ex. Porbeagle. 



6. Alopecidce. Ex. Fox Shark. 



7. Scymniidce. Ex. Greenland Shark. 



8. SquatincB. Ex. Monk-fish. 



9. Zygaznida. Ex. Hammer-head Shark. 



Suborder III. Batides. (Rays, branchial apertures inferior.) 



Fam. 1. Pristidce. Ex. Saw-fish. 



2. Rltinobatidce. Ex. Rhinobates. 



3. Torpedinidce. Ex. Electric ray. 



4. JRaiidce. Ex. Skate. 



5. Trygonidce. Ex. Sting Ray. 



6. Myliobatidce. Ex. Eagle Ray. 



7. CeplialopteridcB. Ex. Cephalopterus. 



1 Kestra, a weapon ; phew, I bear. Many extinct species of this group are known 

 only by their fossil weapons, called ' Ichthyodorulites.' 



