CLASS I 



PISCES 



structures. They arise as folds of skin, which gradually become strengthened 

 by numerous horny fibres (Selachii, Dipnoi). In most Ganoids and Tele- 

 osteans these fibres are replaced by bony rays, which 

 consist of two closely apposed halves. In the Acan- 

 thopterygii (Fig. 11) each half consists of a single 

 piece ; in the Malacopterygii the rays are subdivided 

 by transverse sutures into numerous little pieces and 

 often branch towards the end. Spiny and articulated 

 rays frequently occur in one and the same fin, and in 

 this case the spines are always in front. The rays of 

 the unpaired fins are borne as a rule on flat cartila- 

 ginous or bony supports with which they articulate 

 (Fig. 11, C). These^ fin supports penetrate between 

 the spinous processes of the vertebrae and are known 

 as axonosts or (according to their position) as inter- 

 neurals and interhaemals. 



In many Selachians (Pristiophorus, Raja) every 

 gradation can be observed between the Placoid scales 

 of the external skin and the teeth ; and as both 

 exhibit essentially the same histological structure, 

 while the teeth only assume a firm connection with 

 the head bones in the higher fishes, the teeth must 

 indeed be regarded as dermal structures. 



Among fishes all 



the cartilages or 



bones surround- 

 ing the mouth and gill cavity may 

 bear teeth. They originate in the 

 calcification of skin papillae, con- 

 sisting of a crown wmich freely 

 projects, and of a root which is 



surrounded with 

 or united with 

 Their form and 



Fig. 11. 



A, B, Rays from the 

 dorsal tin of an Acan- 



thopterygian. C, Fin 

 support or inters pi n- 

 ous bone. 



connective tissue 

 the head bones, 

 proportions are 

 extraordinarily variable according to 

 their functions. All gradations exist 

 from the minute granular denticles of 

 the perch to the long brush-like teeth 

 of the sheat-fish, to the robust pre- 

 hensile teeth of the pike and the 

 powerful conical or laniary teeth of 

 Dendrodus or Portheus. Among the 

 sharks there often occur teeth shaped 



like an arrow-head, flattened on one side, with or without 

 lateral denticles. To crush the food there are sometimes 

 obtusely conical teeth, sometimes bean-shaped, hemispherical, 

 or pavement-like teeth, and in certain Selachii the teeth are 

 closely pressed together into a continuous mosaic. Peculiar 

 plates of considerable size occur in Diodon, 



Fia. 10. 



Dorsal fin spine of Hybodus. 

 Upper Lias ; Boll. Wiirtemberg. 



cutting dental 

 Chimaera. 



The number of the teeth in fishes is also as varied 

 While the Dipnoi, Chimaeroidei, and Gymnodonts possess 



Arthrodira, and 



as their shape, 

 onlv four to 



