268 PALEONTOLOGY OF NEW YORK. 



mm., crossed by fine, irregular, lamellose lines of growth. The internal 

 mould retains the impression of the strong longitudinal, surface striae. 



A specimen, consisting of a portion of the chamber of habitation, and 

 about seventeen of the preceding air-chambers, has a length of 100 mm., and 

 i- imperfect at both extremities. 



This species is separated from 0. jaculum on account of its shallower air- 

 chambers, more excentric siphuncle, and longitudinal surface striae. 



Formation and locality. In limestone of the Upper Helderberg group, from 

 \\ <-<tern New York. 



Orthoceras rudiccla. 



PLATES XXXVII, FIG. 2; (XII, UliS. 1,2. 



Ortbocerax rudlcida, Hall. Illustrations of Devonian Fossils: Cephalopoda. Explanation of plate 37, 



fig. 7. 1876. 



Shell straight, rapidly enlarging. Transverse section subcircular. Apical 

 angle about 11°. Initial extremity unknown. 



Chamber of habitation not observed. Air-chambers increasing in depth 

 from the apex to the outer chamber, and varying from three to seven mm. 



Septa with a concavity equal to an arc of 115°. There is a large, elevated 

 areola around the insertion of the siphuncle, which becomes more prominent 

 toward the apex, and is surrounded and covered by a minute mammillary 

 deposit of organic matter. 



Siphuncle subcentral, expanding between the septa ; the walls are very 

 thin, and are obliterated or absorbed in the majority of the air-chambers. 

 Its diameter at the septa is about two mm. where the tube has a diameter 

 of twenty-four mm. 



Test and surface ornamentation unknown. 



The specimen embraces about twenty air-chambers, and has a length of 

 100 mm. 



The air-chambers have been separated and displaced by compression, and 

 the external walls in the cast have become concave. The appearances of 



