ORAL CIRRI OF SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 33 



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 statement 

 in my preliminary paper that no traces of mental tentacles occur in 

 them. Gegenbaur describes in a few Selachii small cartilaginous blocks 

 below the Meckelian cartilage, and these he considers to be rudiments 

 of rays showing that the lower jaw once bore a gill. In accordance 

 with my theory I consider thern to be the rudiments of the mental and 

 submandibular tentacles. Gegenbaur goes on to make the far reaching 

 suggestion that on such cartilages the jugular plates of Crossopterygia 

 may have arisen. On my view the jugular plates would, like the 

 premaxillae and maxillae, arise in connection with tentacles. However 

 direct evidence is still wanting, unless indeed certain phenomena in 

 Geratodus may be interpreted as such. Huxley has described an 

 ensheathing bone at each side of the symphysis, on the ventral face of 

 the mandible. This he takes to be the dentary element, setting aside 

 Giinther's determination of the tooth as the dentary. The bone in 

 question however lies in front of and below the mental cartilage, and 

 may be interpreted on Gegenbaur's suggestion, as a paired anterior 

 jugular plate. Jugular plates occurred in fossil Dipnoi but are usually 

 stated to be absent in the living forms (Smith Woodward). This bone 

 is absent in ProP>pterus. 



The mental tentacle in Myxinoids is represented by a hard root- 

 piece, bearing a rudimentary tentacle and suspended only by ligaments 

 and muscles. It is fully described by Mtiller as the cartilage in the 

 4 th or lowest tentacle. 



Nerve Supply. 



The nerve supply of the mental region is from the R. mentalis and 

 the R, mandibulars externus. The Ramus mentalis in Siluroids runs 

 outside or above the mentomeckelian process and forward, to run down 

 outside the tentacle, wmere that is present, or to branch in the skin, 

 when the tentacle is absent. The Ramus mandibularis externus may 

 be a dissociated branch of the R. mentalis. It runs outside the coronoid 

 process supplying in Auchenaspis the fold of skin below the mental 

 block of cartilage. In Callichthys it is placed not so far forward, 

 c 



