2 78 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



long and slightly curved, lying directly behind the eye. It is fol- 

 lowed by the third, almost straight and shorter than the second ; the 

 fourth, fifth and sixth are straight rod-like bones, longer than the 

 first or third, the sixth being the shortest of the three. All are trav- 

 ersed by a channel in which lies a mucous canal, more or fewer pos- 

 sessing an opening by which the canal communicates with the ex- 

 terior. 



17. Adnasal, PI. II., Fig. 1, An.) 



A small bone on either side, lying at the base of maxillary tentacle 

 in the fascia covering the nasal region. It is really a continuation 

 forwards of the infraorbital chain, containing the same mucous canal, 

 which opens by a pore on its surface. The bone is slightly triangu- 

 lar, with curved edges, the apex being directed forwards. 



18. Nasals, (PI. II., Fig. 1, Na.) 



Are small bones in Amiurus, lying on either side between the ad- 

 nasal and the mesethmoid. They are oblong in shape, and are trav- 

 ersed by a channel for a mucous canal which opens by a poi'e on the 

 outer edge of the bone. 



On comparison with other Teleostean crania, the almost entire 

 absence of cartilage is a very noticeable feature. Ossification has 

 progressed so far in every part that it has replaced the original carti- 

 lage entirely, except in the mes- and ectethmoid. Since the cartila- 

 ginous stage precedes in the ontogeny the osseous stage, one must 

 conclude that a form whose skull is completely ossified is phylogene- 

 tically older than one whose skull contains a considerable amount of 

 cartilage, and, therefore, Amiurus and the Siluroids in general form 

 a highly specialized group, which indeed other points in their ana- 

 ■tomy also show. The absence of a canal for the orbital muscles would 

 also appear to characterize only highly specialized types. It is found 

 in forms in which much of the original cartilage persists, but in this 

 form only a rudiment of it is present, indicating its presence in the 

 ancestral forms of the Siluroids. Vrolik 1 mentions a fact in con- 

 nection with the absence of the canal which receives confirmation in 

 Amiurus, namely, that when such is the case, the peti-osum (prootic) 

 is not pierced by the facial and trigeminus (Gadus, Silurus and 

 Lophius). 



1 Vrolik. — Studien iiber die Verknoelierung u. d. Knochen des Sehadels d. Teleostei. Niednr- 

 and. Arch. f. Zool.— Bd. I., 1873. 



