256 
PALEONTOLOGY OF NEW YORK. 
the siphuncle, extending as an ovate expansion to the ventral margin; the 
whole surrounded by concentric striae, which are often continued in a low, 
broad ridge over the cast of the ventral walls of the chambers. Occasion¬ 
ally this ornamentation is obscured and obliterated by an organic deposit, 
consisting of a coarse, mammillary mass, surrounding the expansion extend¬ 
ing to the margin; and a similar deposit, but composed of finer globules, is 
continued over the septa and the exterior walls. This deposit presents the 
same structural appearance as that in 0 . luxum and 0 . oppletum. 
Siphuncle small, cylindrical, slightly excentric, nearer the ventral side, 
indenting the septa. The diameter at the septa is 2.5 mm. 
Test very thin, rarely preserved, having a thickness of .5 mm. Surface 
marked by fine, sharp, transverse striae, of which there are ten in the space 
of two mm. 
The appearances of the internal mould are usually remarkably character¬ 
istic. Specimens not subjected to compression might be mistaken for 0. 
tetricum; but the ornamentation of the septa is a distinguishing feature. 
The compressed specimens present a decidedly exsolute appearance, as may 
be seen by reference to the figures of the species on plate 81. The chambers 
are often partially separated and variously distorted, leaving the walls concave 
and convex in the same individuals, and frequently becoming subsplierical 
or unsymmetrical. 
One individual, preserving a small portion of the chamber of habitation, 
and eight adjacent air-chambers, has a length of 150 mm., with diameters of 
twenty-seven and twelve mm. respectively at the two extremities. Another 
fragment, of twelve chambers, has a length of 120 mm. 
This species is distinguished from other similar forms by the ornamentation 
on the septa. It is further separated from 0. tetricum by its somewhat smaller 
size. The chambers are deeper and more regular than in O. medium and 
0. stylus, and it has not the narrow carina of the former. 
M. Bakkande figures and describes several species having the septa orna¬ 
mented in a measure like the present species; notably, 0. cruciferum, Barr., 
and 0. bonum , Barr. (Syst. Sil. du centre de la Boheme, vol. ii, text 3, pp. 123, 482). 
