S2 FISH GALLERY. 



adult life. The family ranges from the Lias to the Lower 

 Cretaceous, and includes the genera Pachycormus, Euthynotus, 

 Hypsocorrnus, and some others. 



NEICHTHYES or TELEOSTEI 

 (" Modern " Bony Fishes) . 



The great majority of living fishes, fresh-water and marine, 

 fall under the head Neichthyes or Teleostei- The skeleton of 

 the paired fins consists almost entirely of ossified fin-rays of 

 dermal origin, the row of ossified cartilages (pterygia) with which 

 the basal ends of these are connected being very greatly reduced ; 

 except in the reduction of the pterygials the paired fins resemble 

 those of the previous order, the Astylopterygii, and by some 

 authorities the orders Astylopterygii and Neichthyes are grouped 

 together under the same head, the Actinopterygii, characterised by 

 the importance of the dermal fin-rays in comparison with the 

 basal parts of the pectoral fin-skeleton. The muscles of the fin 

 are confined to the basal parts, so that there is no " lobe " to 

 the fin such as occurs in the order Stylopterygii. 



The scales are thin, oval and overlapping ; in a few cases they 

 are absent, or exist in the form of hard scutes. Only in extinct 

 transitional forms such as Pholidophorus are rhombic scales with 

 superficial layer of ganoin present, and the same applies to the 

 fulcra of the fins. The skeleton is mainly bony ; the vertebrae are 

 distinct, and the centra are usually hollow in front and behind 

 (ampbiccelous). In most cases the hind end of the vertebral 

 column is uptilted, although the outline of the tail fin is 

 symmetrical (homocercal) with an abruptly truncated or forked 

 hind edge. The vomerine bone is single, never paired ; there is 

 no splenial bone in the mandible and no exact equivalent to 

 the coronoid bone of the Astylopterygii ; there is a supraoccipital 

 bone in the cranium. The infraclavicular bone of the shoulder- 

 girdle is wanting. 



The spiracle is closed. The heart has no conus arteriosus 

 (except Albula) ; the intestine has no spiral valve (except Chiro- 

 centrus) ; the optic nerves cross one another beneath the brain 

 and do not form an optic chiasma. 



