282 PALEONTOLOGY OF NEW YORK. 



Test strong, having a thickness of from one to two mm. on the chamber 

 of habitation. Surface marked by very fine, minutely undulating irregular, 

 laniellose lines of growth, crossed by indistinct, longitudinal striae. The 

 ventral side is indicated by a broad sinus in the aperture and surface- 

 markings, and a carina on the cast of the interior. 



The internal mould of the chamber of habitation is made characteristic by 

 the constrictions of the tube, and the abrupt contraction of the aperture. 

 These features of the interior are less conspicuous on the exterior of the shell. 



An individual, consisting of the chamber of habitation entire, and about 

 thirteen attached air-chambers, has a length of 115 mm., with a diameter 

 near the aperture of over forty mm. 



This species is distinguished from 0. Marcellense by its more gradually enlarg- 

 ing tube, its much shorter and more characteristic chamber of habitation, the 

 position and size of the siphuncle, the depth of the chambers, and the ornamen- 

 tation of the test. It was formerly not distinguished from 0. Marcellense ; but 

 an examination of all the characters has shown the very unlike features pos- 

 sessed by this species. The want of sufficient material precludes any compari- 

 sons based on the organic deposit, which is probably present in this species, 

 as indicated by the ventral carina on the internal mould of the air-chambers. 



Formation and locality. In the Goniatite limestone of the Marcellus shale at 

 Schoharie, N. Y. 



Orthoceras aptum, n. sp. 



PLATE XXXVIII, FIG. 8. 



Orllutcerat tyyum (Sabmanx), Hall (in part). Illustrations of Devonian Fossils: Cephalopoda Kxplanation 



of pi. 38, fig-. 8. 1870. 



Shell straight, regularly and gradually enlarging from the apex to the aperture. 

 Transverse section circular. Apical angle 5°. Initial extremity unknown. 



Chamber of habitation having a length of about three times the diameter at 

 the last Beptum, expanding slightly beyond the general enlargement of the 

 tube, with a very gentle constriction near the middle, which is shown on the 

 cast of the interior. Chambers numerous, regularly increasing in depth 



