256 PALEONTOLOGY OF NEW YORK. 



the siphuncle, extending as an ovate expansion to the ventral margiiv; 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 subspherical 

 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 0. medium and 

 0. stylus, and it has not the narrow carina of the former. 



M. Barkande figures and describes several species having the. septa orna- 

 mented in a measure like the present species; notably, O. cruciferum, Barr., 

 and 0. bonum, Barr. (St/st. Sil. du centre de la Boheme, vol. ii, text 3, pp. 123, 482). 



