ORAL CIRRI OF SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 25 



Callichtliys (Figs. 5, 6 and 7). They are supported, through the inter- 

 mediation of a small maxillary bone (os labial Cuvier, adnasal 

 McMurrich), on the prepalatine piece and extend downwards and out- 

 wards. They are also present in Cypr'midae. The coronoid tentacle is 

 most typically shown in Trichornycterus, where it is very long. Its base 

 passes continuously into a large procartilaginous root piece, the 

 coronoid piece, which is firmly attached to the cartilaginous coronoid 

 process of the lower jaw. 



In Callichthys the tentacle is fused, near its base, with the mental 

 tentacle, and the coronoid piece is only represented by a small mass of 

 procartilage. The rudimentary tentacles of Rypostomidae are maxillary 

 tentacles. 



The tentacle at the angle of the mouth in the majority of Siluroids 

 combines the characters of the two tentacles, and may thus be termed 

 maxillo-coronoid. In Auchenaspis this is well shown (Figs. 1 and 2). 

 The coronoid piece is seen to be continuous with the tentacle, but on 

 the other hand this tentacle is borne by the maxilla, which articulates 

 with the prepalatine piece. In Silurus (Figs. 3 and 4) the coronoid 

 piece does not reach the tentacle actually, but approaches near it. 

 What I think may be regarded as the proof of the fusion of maxillary 

 and coronoid tentacles is given by the nerve supply. What occurs in 

 the case of the mental and coronoid tentacle of Callichthys gives a clue 

 as to how the fusion may have actually taken place. Judging from 

 the nerve supply the outer pair of barbels of the Sturgeon are maxillary 

 tentacles. 



The prepalatine piece requires careful observation. In the young 

 Silurus it occurs as a squarish block of cartilage, articulating with the 

 preorbital process. In my specimen of Auchenaspis ossification had 

 set in around the articulation, and consequently only apophyses of 

 cartilage are left. The anterior block always lies very far forward 

 in the snout. The posterior end serves for the attachment of the 

 adductor muscle proceeding from the ethmoid wall. The muscle is 

 mentioned by Stannius. It works the maxillo-coronoid tentacle. In 

 Trichornycterus a small cartilage remains in the posterior part of the 

 articulation, and on comparing closely the sections of the young 

 Callichthys, I found that a small projection of cartilage from the skull 

 represented this free cartilage of Trichornycterus. Thus we have a 

 stage when the prepalatine cartilage is continuous with the skull 

 cartilage (the spot is marked with an asterisk in Fig. 7). 



