356 



of the Trigeminus by maxillary and coronoid branches but these 

 branches interchange fibres. 



In Auchenaspis one tentacle appears to represent the two fused, 

 for the coronoid piece is continuous with a tentacle which possesses 

 all the characters of a maxillary tentacle, that is in its relation to 

 the maxillary bone and supply from maxillary nerve. This fusion has 

 probably also taken place in Silurus, In Callichthys, as will be shown, 

 tentacles do fuse and this process is of interest as perhaps explaining 

 the reduction in number of tentacles from Amphioxus to Myxine. 



A coronoid „labial" is present in Polypterus and probably the 

 lower labial of Selachii is a coronoid piece. Following Cuvier I regard 

 the two upper labials of Selachii as premaxillary and maxillary carti- 

 lages. 



The remaining tentacles of Siluroids are those of the lower jaw. 

 The fourth is the mental and is situated in front of or at the sides 

 of the symphysis of the lower jaw. It is well developed in Auchen- 

 aspis, Silurus and Callichthys and no doubt in many others but is 

 very variable. In Auchenaspis a portion of the rootpiece has become 

 modified into a huge block of precartilage supporting a fold of the 

 lower lip. In Silurus the rootpiece has been lost and the tentacle is 

 sustained by a secondary superficial plate of precartilage. In Callich- 

 thys the bases of the tentacles unite and the fused portions lie in 

 front of the symphysis of the dentary bones while distally the tentacle 

 of each side fuses with the coronoid tentacle, a point referred to above. 

 Callichthys is interesting because the fused part of the mental root- 

 pieces bears a striking similarity to the corresponding structure in 

 Protopterus which has been especially well figured by Rose ') only that 

 in the latter animal the block is of hyaline cartilage and passes con- 

 tinuously by a process below and round the dentary bone (tooth of 

 the lower jaw) into the Meckelian cartilage. The unpaired mental 

 rootpiece is represented in Callorhynchus by a huge block of hyaline 

 cartilage (figured by Müller) in front of the lower jaw. In Chimaera 

 only a paired rudiment of this remains (Hubrecht 2 ) and in Selachii 

 it has disappeared entirely. In Selachii indeed it is a fruitless task 

 to search for characters of the earliest vertebrates either in their 

 anatomy or ontogeny. 



1) Rose, lieber Zahnbau und Zahnwechsel der Dipnoer, Anat. Anz., 

 Bd. VII, 1892. 



2) Hubrecht, Beitrag zur Kenntnis des Kopfskelets der Holocephalen, 

 Morph. Jahrbücher, Bd. III, 1877. 



