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THE OSTEOLOGY OF AMIURUS CATUS. 279 
As regards the various bones of the skull, they differ in no very 
essential points from those of Silwrws 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, (Pl. IL., 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!' 
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 
1Sagemehl.—Beitrage zur verg]. Anat. der Fische. Das Cranium von Amia Calva. L., Morph. 
Jahrb. Bd. IX., 2nd Heft, 1883. 
