230 THE AMERICAN NATURALIST. 
that presented on a preceding page. If this assumption be 
true, the articular head of the mammalian lower jaw should 
represent the articulare, while between this and the tip of the 
jaw, several bones — angulare, splenial, dentary — should occur. 
In fact, the lower jaw in every mammal, so far as known, ossi- 
fies as a single membrane bone, which we prefer to regard, until 
better evidence is forthcoming, as the dentary of the lower ver- 
tebrates. The articulare we recognize in the body of the mal- 
leus, while since a new fulcrum of the lower jaw has been formed, 
the other bones, angulare, supra-angulare, splenial, etc., having 
no longer cause for existence, have disappeared without leaving 
a trace behind. Their proper position would be around that por- 
tion of Meckel’s cartilage which for a while persists between 
the malleus and the osseous lower jaw. 
In examining the shape, relations, etc., of the manubrium, 
which, as was noticed above, arises separately from the rest of 
the malleus, one can hardly fail to be struck with its resem- 
blance to a somewhat reduced visceral arch. This resemblancé 
is the more interesting since the cartilage appears in the very 
place where, according to several students of the problem of the 
segmentation of the vertebrate head (Dohrn, van‘ Wijhe, Beard, 
Locy, Neal, etc.), a segment has apparently almost entirely dis- 
appeared from the vertebrate head. Whether this and the distal 
cartilage of the reptilian columella be the visceral arch of the 
missing segment, and whether the Eustachian tube really rep- 
resents two confluent gill slits, we do not at present care to 
discuss. 
