ORAL CIRRI OF SILUROIDS AND ORIGIN OP THE HEAD IN VERTEBRATES. 41 



occur so far as I know in other tadpoles nor in the larvae of Urodeles. 

 Therefore probably we have here a case of larval reversion, but it is 

 only an exceedingly incomplete reversion. 



The Zoological Position of Siluroids. 



The Siluroids are mostly freshwater fish with an extraordinary 

 diversity of habits and structure and with a remarkably wide geo- 

 graphical distribution. While however the group as a whole occurs in 

 all the zoogeographical regions, yet certain families are confined to one. 

 Thus, for instance, the Loricarina with peculiar anatomical features 

 appear to be confined to the rivers of S. America. (Certain forms from 

 the oriental region have been allied with them by some ichthyologists.) 

 They are freshwater forms without any means of passing across seas, 

 being heavily armoured with feeble powers of swimming, lying at the 

 bottom of pools in the daytime, and creeping about at night on banks 

 by means of their strong spines, and feeding on soft substances more 

 or less putrified (Weyenbergh). 



We are justified by the pi'inciples of geographical distribution in 

 atti-ibuting to Loricarina an antiquity like that of Lepidosiren. 



Nor is there any reason, as far as distribution is concerned, for 

 denying to other families, such as the Clariina, an immense antiquity. 

 They are a now flourishing group, while the Dipnoi, with restricted 

 range are a decadent group. 



The dermal skeleton of Hypostoma and Callichthys has been 

 exhaustively investigated by Hertwig, who found that their dermal 

 teeth are homologous with the placoid scales of Elasmobranchs. 

 Therefore the dermal skeleton must be regarded as exceedingly primi- 

 tive. From that of Hypostoma may be derived that of Acipenser, 

 which, however, is considerably more modified. Recently Klaatsch has 

 attempted to upset Hertwig's conclusions, but I am not alone in 

 thinking that in spite of the technical excellency of Klaatsch's work, he 

 has signally failed to prove his point. 



Another feature of considerable interest is revealed by the oral 

 teeth of Hypostomidae. Some forms have been excellently figured by 

 Kner. 1 would specially refer to his Fig. 1, Tab. 5. 



The bent hook-like teeth of the premaxillary and dentary bones all 

 converge in the same direction, and the two premaxillae and the two 



