THE DIPNOI. 167 



The two remaining groups are very peculiar ; but I con- 

 fess I do not see upon what ground they can be regarded as 

 of ordinal value. 



5. The Lophobranchii.— The body is covered with bony 

 plates. The ventral fins are almost always absent. The 

 inferior pharyngeal bones ai-e distinct. The branchial pro- 

 cesses have a clavate form, being larger at the free than at 

 the attached ends, and are in this respect unlike those of 

 any other fishes. (Pegasidce, Syngnathidce.) 



6. The Pledogncdhi.—The body is covered with plates or 

 spines. The ventral fins are absent, or represented only 

 by spines. The inferior pharyngeal bones are distinct. 

 The premaxiUse and. usually, the hyomandibular, are im- 

 moveably united with the skull — a character of rare occur- 

 rence among other fishes. {Gymnod&ntidce, OstracionUdce, 

 Balistidce.) 



The greater number of Teleostei are marine. No Aiiacan- 

 thini, Plectognathi, or Loplwhrcmcliii, and only one family 

 of Pharyngognathi (the Chromidce), inhabit completely fresh 

 water. Comparatively few Acanthoiiteri are fluviatile. On 

 the other hand, by far the greater number of the Physostomi 

 are, either temporarily or permanently, freshwater fish. 



If the Lejitolejndce (Thrissops, Leptolepis, Tliarsis) are 

 Ganoids, the Teleostei are not known before the Cretaceous 

 epoch, when both Physostomi and Acanthopteri make their 

 appearance, under forms, some of which (e.g., Beryx) are 

 genericaUy identical with fish living at the present day. 



VI. The Dipnoi.— The " Mudfishes " of the rivers of the 

 east and west coasts of Africa and of eastern South America 

 are nearly transitional forms between the Pisces and the 

 Atnrphibia. 



The eel-like body, covered with overlapping cycloid 

 scales, tapers to a point at its caudal extremity, and is 

 provided with two pairs of long, ribbon-like, pointed ex- 

 tremities, and with a caudal fiji. 



The spinal column consists of a thick notochord, invested 

 by a cartilaginous sheath, without any osseous or cartila- 



