23, ie THE HISTORY OF CREATION, 
The Thrissopide 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 air 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 completely 
separated from the throat. Hence there arose a second 
legion of Osseous fish, the Physoclisti, 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, ete. further, the Lock-jaws (Plectoonathi), Trunk 
fish, and Globe-fish and the Bushy-gills (Lophobranchi), viz., 
Pipe-fish, and Sea-horses. There are, however, only very 
few Physoclisti 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 Mud-fish, 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 different parts 
of Africa, A third large Salamander-fish (Ceratodus Fosteri) 
