212 THE HISTORY OF CREATION. 



The Thrissopidse of the Oolitic period (Thrissops, Leptolepis, 

 Tharsis), which are most closely allied to the herrings of the 

 present day, are probably the oldest of all Osseous fish, 

 and have directly arisen out of Round-scaled Ganoid fish, 

 closely allied to the existing Amia. In the older Osseous 

 fish of the legion called Physostomi, as also in the 

 Ganoides, the swimming bladder throughout life was 

 connected with the throat by a permanent aii- passage 

 (a kind of windpipe). This is still the case with all the 

 fish belonging to this legion, namely, with herrings, salmon, 

 carp, shad, eels, etc. However, during the chalk period this 

 air passage, in some of the Physostomi, became constricted 

 and closed, and the swimming bladder was thus completelj' 

 separated from the throat. Hence there arose a second 

 legion of Osseous fish, the Physodisti, which did not 

 attain their actual development until the tertiary epoch, 

 and soon far surpassed the Physostomi in variety. To this 

 legion belong most of the sea fish of the present day, 

 especially the large families of the Turbot, Tunny, Wrasse, 

 Crowfish, etc., further, the Lock-jaws (Plectognathi), Trunk 

 fish, and Globe-fish and the Bushy -giUs (Lophobranchi), viz., 

 Pipe-fish, and Sea-horses. There are, however, only very 

 few Physodisti among our river fish, for instance. Perch 

 and Sticklebacks ; the majority of river fish are Physostomi. 

 Midway between genuine Fish and Amphibia is the 

 remarkable class of Miid-Jish, or Scaly Sirens (Dipneusta, 

 or Protopteri). There now exist only a few representatives 

 of this class, namely, the American Mud-fish (Lepidosiren 

 paradoxa) in . the region .of the river Amazon, and the 

 African Mud-fish (Protopterus annectens) in difierent parts 

 of Africa. A third large Salamander-fish (Ceratodus Foster!) 



