THE OSTEOLOGY OF AMIURUS CATUS. 279' 



As regards the various bones of the skull, they differ in no very. 

 essential points from those of Silurus glanis, which have been de- 

 scribed in general terms and for comparative purposes only by Vro- 

 lik. All the bones usually found in Teleostean crania are present 

 with the exceptions of the opisthotic, intercalare, and parietals. The 

 principal features are the presence of a well-ossified and large mes- 

 ethmoid ; the orbitosphenoid forming three sides of a canal for the 

 olfactorius, thereby separating widely the eyes and acting as an in- 

 terorbital septum ; the meeting of the prootics at the base of the 

 skull ; and the absence of teeth in the vomer, a point of some im- 

 portance, since certain closely related forms are provided with vom- 

 erine teeth. 



Certain points in the development of the cranial bones merit a de- 

 tailed description. In a young Amiwrus, about 20 mm. in length, it 

 was to be noticed that wherever a mucous canal appeared in trans- 

 verse section a ring of bone surrounded and protected it, (PI. II., Fig. 

 8, MC), so that each of these canals in the cranium was surrounded 

 by an osseous tube. The bone was apparently deposited in mem- 

 brane, and was evidently formed solely for the protection of the mu- 

 cous canal. In certain cases a bone, usually perforated for the emis- 

 sion of a branch from the canal to a pore, became formed by a lateral 

 extension of this osseous tube into the adjacent connective tissue. 

 Instances of such bones are the infraorbital chain, the adnasals and 

 nasals. The adnasals in reality, then, as was stated above, belong to^ 

 the same group as the bones of the infraorbital chain, and may be 

 described as the anterior ossicle of that chain, since it is formed in 

 the same manner, and is traversed by the same canal. Sagemehl 1 

 proposes to name it the antorbital, but, since its function is not only 

 to protect the enclosed mucous canal but also to protect the nasal 

 region to which it stands in the same relation as does the nasal, I 

 prefer the name employed. 



In the majority of cases, however, the osseous tube does not remain 

 distinct but fuses with the subjacent bone, whether formed in mem- 

 brane or perichondrally. In the case of the frontals, for instance^ 

 the mucous canal bone unites with the underlying bone formed in 

 membrane, and in the sphenotic and pterotic (Fig. 8) a similar union 

 occurs with the perichondral bone with which the ossification of the 



^Sagemehl.— Beitrage zur vergl. Anat. rler Fische. Das Cranium von Amia Calva. L., Morph. 

 Jahrb. Bd. IX., 2nd Heft. 1883. 



