280 
PALEONTOLOGY OF NEW YORK. 
individuals, and over different portions of the tube. There are also slight 
variations in the depth of the chambers. It is distinguished from 0. aptum, 
with which it is associated, by its more distant septa, its greater apical angle, 
and straight, septal sutures. And from O.fustis by the greater development of 
the chamber of habitation, the absence of any decided constriction or abrupt 
contraction at the aperture, the depth of the chambers, the excentric and 
smaller siphuncle, and the surface ornamentation. In many respects it resem¬ 
bles 0. luxum; but the position of the siphuncle, its elements, and the dimen¬ 
sions of the tube, are very different. From the other species in the present 
group, it is distinguished by its proportions, surface characters and the organic 
deposit. 
In this species, as in all the species described in this volume, which have 
developed an organic deposit, the appearances of this deposit depend upon the 
mode of weathering, the conditions of preservation, the material filling the 
cavities, and the manner in which the chambers separate at the septa. The 
elevated areola and its processes may separate from the convex surface of the 
septa, and remain imbedded in the filling of the air-chambers, leaving the septa 
apparently smooth. Or it may remain attached to the convex surface, giving 
to the septa a totally different aspect. From the condition of preservation in 
many of the specimens, the raised areola, which has been considered as a 
deposit, may be explained by supposing the deposition to have been made on 
the concave surface of the septa, from the'margins toward the siphuncle; and 
which has remained attached to the filling of the air-chambers, leaving an 
elevated expansion around the insertion of the siphuncle on the convex side of 
the septa. In support of this view, the areola is always larger and has less of 
an elevation near the chamber of habitation, gradually diminishing in diameter, 
and becoming more prominent toward the apex, where the amount of organic 
deposition is at the maximum. Whatever may have been the original purpose 
of the deposit, the shells in which it is more fully developed always present 
the appearance of deterioration or disease. 
The markings on the ventral walls of the air-chambers, as described, are 
evidently the cast or the reverse of the original conditions. These suggestions 
