-SALMONS AND HERRINGS. 83 



Physostomi. 



The Teleostei or Neichthyes are divided into two main sections 

 or " grades/'' the Physostomi and the Physoclisti. In the fishes 

 of the grade Physostomi the air-bladder is usually present, and 

 remains in open communication with the alimentary canal by a 

 tube called the " pneumatic duct." The fishes are, on the whole, 

 less specialised than those of the grade Physoclisti. Included 

 in the Physostomi are the Salmoni-clupeiformes or Isospondyli — 

 the Salmons, Herrings aud allied fishes ; the Cypriui-siluriformes 

 or Ostariophysi — the Carps and Cat-fisbes ; the Anguilliformes or 

 Apodes — the Eels ; and the Esociformes or Haplomi — the Pikes 

 and their allies. 



Salmoni-clupeiformes (Salmons, Herrings, etc.). 



The Salmoni-clupeiformes or Isospondyli are the most primitive 

 of the Teleostei, aud approach the Ganoid fishes (e. g. Amia) more 

 closely than do other Teleosteans. The suborder includes the 

 Tarpons, Herrings, Mormyrids, Osteoglossids, Salmons and 

 Trouts. The anterior vertebra? are simple and not converted 

 into a mechanical link-work for connecting the air-bladder with 

 the ear as they are in the next suborder, the Cyprini-siluriformes. 

 The maxillary bone forms usually part of the margin of the upper 

 jaw ; there are no barbels. The pelvic fins have many rays and 

 are abdominal in position, i. e. are set some distance behind the 

 pectoral fins. The shoulder-girdle is connected with the back 

 of the cranium by the post-temporal bone, and has a mesocoracoid 

 element (see girdle of Salmon, 277), a bone found in the pectoral 

 girdle of all Ganoids, but among the Teleostei in the Salmoni- 

 clupeiformes and the Cyprini-siluriformes only. None of the fins 

 have spinous fin-rays. 



The families Leptolepidse (e. g. Leptolepis dubius, 232), Pholi- 

 dophoridse and Oligopleuridse include small fishes which occupy 

 a low position among the Neichthyes, and connect them with the 

 Amioid fishes. These fishes are extinct, and their remains are 

 found in Mesozoic strata. In all of them there is an aperture in 

 the middle of each centrum through which passed the more or less 



g2 



