CEPHALOPODA. 289 



The internal mould of the chambers is smooth, with the sutures but little 

 impressed. 



One fragment, comprising a portion of the outer chamber and twenty-one 

 of the attached air-chambers, has a length of eighty-five mm. A septate 

 fragment, retaining nearly its normal form, measures fifty-three mm., with 

 diameters of seventeen and eleven mm. respectively, at the two extremities. 

 The chamber of habitation has a length of "seventy mm. in one individual, 

 with a diameter of about fifteen mm., and is apparently not entire. 



This species is distinguished from 0. subulatum by its constricted and much 

 more developed chamber of habitation. In the depth of the air-chambers and 

 the position of the siphuncle the two species are very similar. It is farther 

 removed from 0. exile by the depth of the air-chambers and its central 

 siphuncle. The constriction is also generally broader and nearer the middle of 

 the chamber of habitation, sometimes becoming nearly as prominent a feature 

 as represented in Vanuxem's original figure of the species. 0. emaceratum may 

 be readily distinguished by the much greater depth of the air-chambers. 



Mr. Conrad* gave this name to an annulated form, which has not since been 

 recognized. The absence of an illustration and the want of sufficient definition 

 in his description renders it undesirable to change the present recognized 

 species of Yanuxkm. 



Formation and localities. In the Hamilton group, in the counties of Ontario 

 and Madison, N. Y. ; and at Cumberland, Md. 



* Mr. Conrad's description is as follows: 



<i it It octroi a/iuit riclum. — Shell smooth, tapering gradually, with a few remote transverse furrows. 

 Locality, town of Madison, Madison county. (Geolog. Surv. N. Y. : Pal. Dep., Annual Report, p. 117. 1838.) 



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