300 PALEONTOLOGY OF NEW YORK. 



Septa smooth, with a concavity equal to more than the depth of the adja- 

 cent air-chambers. Sutures straight and horizontal ; essentially correspond- 

 ing to the annulations. 



Siphuncle subcentral, moniliform, slightly expanding ; having a diameter 

 of 1.5 mm. at the septa, where the tube has a diameter of ten mm. 



Test very thin, rarely preserved. Its thickness is .2 mm. where the tube 

 has a diameter of fifteen mm. Tube ornamented with regular, numerous, 

 horizontal annulations. Surface marked by regular, fine, thread-like, longi- 

 tudinal striae, which are crossed by finer, less prominent lines of growth. 

 The longitudinal striae are usually continuous, but are occasionally inter- 

 rupted by the lines of growth, of which there are about twelve in the space 

 of two mm. 



On the internal mould the striae are usually impressed, but they appear as 

 if rounded and continuous, and do not present the sharp, thread-like char- 

 acter as on the exterior. 



A large fragment has a length of 140 mm., and shows forty-two annula- 

 tions. Portions of smaller individuals show from four to six annulations in 

 the length of ten mm., varying with the diameter of the tube. 



This species is distinguished from O. crotalum by its more frequent and less 

 prominent annulations, and by the somewhat coarser longitudinal striae. It 

 differs from 0. ccdamen in not having the longitudinal striae rounded and often 

 interrupted by lamellose lines of growth ; and from 0. Union and 0. Thestor by 

 its much less frequent annulations and different surface characters. 



The sections illustrated on plate 82, figs. 14, 15, are of specimens which 

 occur in the calcareous layers of the Hamilton group at Pratt's Falls, Onon- 

 daga county, in association with undoubted 0. nuntium. These specimens 

 show the expanded character of the siphuncle and other internal features, 

 which have not been fully observed in other individuals of the species. 



Several fragments have been obtained from collections made in the Upper 

 Helderberg limestones in Canada West, which possess the general form and 

 surface characters of this species. The material, which is too meagre for satis- 



