THE PISCES. 113 



rudimentary external branchiae and functional, pectinated, 

 internal branchiae. 



This order contains only the " Mud-fish," Lepidosiren and 

 RhinocrypUs. 



2. The ELASMOBEANCHII. The skull and mandible are well 

 developed, but there are no cranial bones. 



The condition of the vertebral column varies. 



There are two pair of fins, each supported by many series of 

 cartilages. The posterior pair are placed close to the anus. 



The pectoral arch has no clavicle. 



The heart has one auricle, and a rhythmically contrac- 

 tile bulbus arteriosus, which contains striated muscular fibre 

 in its walls, and is provided with several transverse rows of 

 valves. The gills are pouch-like. The optic nerves form a 

 chiasma. 



To this order belong the Sharks, Bays, and Chimoerse. 



3. The GANOIDEI. The skull has cranial bones, and there is 

 a mandible. The condition of the vertebral column varies. 

 There are usually two pair of limbs of the same essential struc- 

 ture as those of the Elasmobranchii. The pectoral arch has a 

 clavicle, and the posterior limbs are placed clo-ie to the anus. 



The heart and optic nerves are as in the Elasmobranchii. 



The gills and the opercular apparatus are as in the Te- 

 leostei. This order includes the Sturgeons and bony Pikes 

 (Lepidosteus), besides Polypterus, Amia, and numerous extinct 

 genera. 



4. The TELEOSTEI comprises the majority of existing genera 

 of fishes provided with a bony skeleton. The skull has cranial 

 bones and a mandible. More or less ossified distinct vertebrae 

 are always present. The limbs, when they exist, have the same 

 general structure as those of the Ganoids, but the ventral fins 

 vary in position. The pectoral arch has a clavicle. 



The gills are pectinated or tufted ; a bony operculum and 

 pre-operculum and branchiostegal rays are always developed. 



The bulbus aortse is not rhythmically contractile, and it is 

 separated from the ventricle by only a single row of valves. 



The optic nerves cross, but do not form a chiasma. 



5. The MAESIPOBKANCHII includes the Lampreys and Hags. 



