27 J PALEONTOLOGY OF JNEW YORK. 



Section a. — Robust forms. 



Orthockras Eriense. 



I'l . vtks \i.. nos. 1-4; i.xxxvn, kk;s. i, 2. 



Ortltoctras robn-ttum. Hall. Illustrations of Devonian Fossils': Cephalopoda. Explanation of pi. 40. IS76. 

 N ' timet rtut robustum. Wijtchkll. Am. Jour. Sci. -Arts, 2d series, vol. 33. 1862. 



" " Barium.i:. syst. Silur. da centre de la Bohcme. 1866. 



" Eriense, Hall. Catalogue Am. Pal. Fossils, S. A. Miller, p. 174. 1877. 



Shell very large, straight, robust, regularly enlarging to the aperture. Trans- 

 verse section circular. Apical angle 8°. Initial extremity unknown. 



Chamber of habitation well developed, having a length equal to twice its 

 transverse diameter through the centre, with a diameter at the last septum 

 of three-fourths the diameter at the aperture. The tube regularly expands 

 to the aperture, which is slightly contracted. Aperture entire, with a gentle 

 ret ral curve on the ventral margin ; margins acute. Air-chambers regular, 

 numerous, having a depth of from ten to fifteen mm., gradually becoming 

 more numerous toward the apex. The internal cast of the walls is smooth 

 and flat, with a minutely longitudinal, striate band around the anterior mar- 

 gin of each air-chamber, with more irregular, slightly undulating, concentric 

 BtriaB over the central portion 



Septa thin, smooth, having a concavity equal to an arc of 116°, or a depth 

 greater than that of the chambers. Sutures straight and horizontal. 



Siphuncle large, straight, cylindrical, with a diameter of eight mm. where 

 the tube has a diameter of seventy-five mm. 



The test has a thickness of nearly one mm. on the chamber of habitation. 

 Surface cancellated; marked by regular stria} of growth, interrupted by fine 

 longitudinal striae. 



The internal mould is essentially smooth, with the septal sutures but 

 slightly indented. The very fine, longitudinal and transverse striae over a 

 portion of the chamber walls are characteristic but not prominent. 



A specimen preserving an apparently entire chamber of habitation, and 

 two of the adjacent air-chambers, has a length of twenty-one centimetres, 



