﻿354 



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 



HHBflnrnniDftr- 



Fig. 1.— Nautilus nmbilieatus. 



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, 



