The oral cirri of Siluroids and the origin of the head in Vertebrates. 409 



is shown in the proximal part of the maxillary tentacle in Tricho- 

 mycterus (Kig. 5, left side of tlie drawing). 



The outer prong of the bifurcation may have expanded second- 

 arily so as to have formed this great block. The outer mental ten- 

 tacle of Misyurnus is a continuation from a corresponding block just 

 as if the backward projection of the piece in Äuchenaspis were pro- 

 longed into a tentacle. A corresponding piece is found in Motella 

 tricirrata, and no doubt in many other Teleostei, in fact this may be 

 in some tish the "Mundwinkelknorpel" of Stannius. 



A median unpaired mental tentacle is also present in Motella and 

 Gadidae^ but it shows few of the characters of a typical mental 

 tentacle. This tentacle is paired in MuUus and Upeneus. 



The lower support of the velum may be a derivative of the mental 

 tentacle though much modified. The mental tentacles of Callichthys 

 show a remarkable similarity with the cartilage in front of the lower 

 jaw of Protopierus, as figured especially well by Rose and of Ceratodus 

 as figured by Huxley. I have, as in most cases, verified the observa- 

 tions myself, by sections in Protopterus and dissection in Ceratodus. 

 This cartilage however passes under the lower teeth and is continuous 

 with the Meckelian cartilage showing in this respect no correspondence 

 with Callichthys. The position of this cartilage led Huxley, erro- 

 neously I think, to term the lower teeth splenial. 



The huge unpaired block of cartilage in front of the lower jaw 

 in Callorhynchus is obviously a mental piece, corresponding to the 

 mental cartilage of Dipnoi (lower labial of Günther), and somewhat 

 doubtfully to the mental piece of Äuchenaspis. In Chimaera as 

 shown by Hubrecht and Vetter it is represented by a small pair 

 of cartilages below the coronoid labials at each side of the Meckelian 

 cartilage. Howes has compared this mental piece with that of the 

 Myxinoids and with the lower half of the annular cartilage of the 

 lamprey, which Huxley homologised with the lower labial of the 

 tadpole. 



Considering now the Selachii, I have to withdraw a hasty state- 

 ment in my preliminary paper that no traces of mental tentacles 

 occur in them. Gecjenbaur describes in a few Selachii small carti- 

 laginous blocks below the Meckelian cartilage, and these he considers 

 to be rudiments of rays, showing that the lower jaw once bore a gill. 

 In acc(»r(lance with my theory I consider them to be rudiments of 

 the mental and submandibular tentacles. Geuenhaur goes on to make 

 the far reaching suggestion that on such cartilages the jugular plates 



