CHAPTER XXXIV 

 THE CROSSOPTERYGII 



LASS Teleostomi. We may unite the remaining groups 

 of fishes into a single class, for which the name Teleos- 

 tomi (reAeo?, true; aro^ia, mouth), proposed by Bona- 

 parte in 1838, may be retained. The fishes of this class are 

 characterized by the presence of a suspensorium to the man- 

 dible, by the existence of membrane-bones (opercles, sub- 

 orbitals, etc.) on the head, by a single gill-opening leading to 

 gill-arches bearing filamentous gills, and by the absence of 

 claspers on the ventral fins. The skeleton is at least partly 

 ossified in all the Teleostomi. More important as a primary 

 character, distinguishing these fishes from the sharks, is the 

 presence typically and primitively of the air-bladder. This 

 is at first a lung, arising as a diverticulum from the ventral side 

 of the oesophagus, but in later forms it becomes dorsal and is, 

 by degrees, degraded into a swim-bladder, and in very many 

 forms it is altogether lost with age. 



This group comprises the vast majority of recent fishes, 

 as well as a large percentage of those known only as fossils. 

 In these the condition of the lung can be only guessed. 



The Teleostomi are doubtless derived from sharks, their 

 relationship being possibly nearest to the Ichthyotomi or to the 

 primitive Chim&ras. The Dipnoans among Teleostomi retain 

 the shark-like condition of the upper jaw, made of palatal 

 elements, which may be, as in the Chim&ra, fused with the cra- 

 nium. In the lower forms also the primitive diphycercal or 

 protocercal form of tail is retained, as also the archipterygium 

 or jointed axis of the paired fins, fringed with rays on one or 

 both sides. 



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