04 
face, and swimming with a jerky motion; though their open aper- 
tures, as a rule, show their normal condition to have been reptant, 
or bottom-crawling. The exceptional shells, which depart from 
the typical form in the sinus and apertures, exhibit their peculiari- 
ties in the adults, but not, as a rule, in the young, except in cases 
V 
VA 
ZA 
G4 A 
Ly EA 
WY : 
| 
i 
ZS 
i \ 
71 re 
Z /) 
= a LUiy } 
Wf My 
Af pol} 
YY, » | 
tpi ff J) If 
< Yy, iy WY! yp 
LLM YP 
Fig. 1.—Nautilus umbilicatus. 
where direct inheritance has occasioned the exception, and these 
are, in fact, the most conclusive proofs of the power of the habitat 
to produce permanent changes in the apertures. 
The orthoceratitic shells of this order are straight cones, with 
internal septa dividing them into air-chambers, connected by a 
tube passing through all the air-chambers, and opening into the 
body of the animal itself, which occupied a large terminal chamber, 
