316 ORDERS OF FISHES. 



and always communicates with the oesophagus by means of a 

 duct, which is the homologue of the windpipe. The skin is 

 rarely naked, and is mostly furnished with cycloid scales ; but 

 in some cases ganoid plates are present. 



The more important families comprised in this sub-order are 

 the Mnrcenida (Eels), the Clupeida (Herrings), the Pikes 

 (Esocida>), the Cyprinida (Carp, Chub, Barbel, &c.), and the 

 Salmonidoe (Salmon and Trout). Few of these families ap- 

 peared in rocks older than the Eocene Tertiary. The Salmon- 

 ides are only sparsely represented in deposits older than those 

 of the Post-Tertiary epoch. The Cyprinida and Esoddce are 

 both represented in the fresh-water deposits of the Tertiary 

 period, and the Murcenidce appear for the first time in the 

 Eocene. The genus Osmeroides (fig. 279) has been referred 



Fig. 279. i. Bery.v (Osmeroides) Lewftiensfs, a Percoid fish from the Chalk ; 

 2. OtmeroieUs Mantelli, a Salmonoid fish from the Chalk. 



to a position in the neighbourhood of the Salmonidce, and 

 dates from the Cretaceous period. The Clupeoicls, also, 

 make their first appearance in the Cretaceous period. Also 

 in this group are the Sheat-fishes (Siluridce), which are chiefly 

 noticeable because they are amongst the small number of 

 living fishes possessed of structures of the same nature as 



