CEPHALOPODA. 307 



Orthoceras pacator, n. sp. 



PLATE I. XXXIX, FIGS. 1-6. 



Shell straight, regularly and gradually enlarging to the chamber of habitation. 

 Transverse section circular. Apical angle about 6°. Initial extremity 

 unknown. 



Chamber of habitation well developed, having a length of nearly three 

 times the diameter of the tube at the last septum, regularly enlarging to a 

 point near the aperture, where the tube is abruptly constricted, and expand- 

 ing thence to the aperture. Air-chambers numerous, of regular depth 

 from the apex over the greater portion of the tube, but becoming more fre- 

 quent and irregular toward the outer chamber. Their depth varies gradually 

 from two to four mm. in the length of eighty mm. At the base of the grand 

 chamber there are usually several very shallow air-chambers, which occa- 

 sionally show great inequality in the depth of adjacent chambers. 



Septa smooth, thin, with a concavity equal to an arc of about 103°. Sutures 

 straight and horizontal in specimens not disturbed by compression. 



Siphuncle excentric, distant from the nearest point on the septal margins 

 about one-fourth the diameter of the tube, having a diameter at the septa of 

 two mm. where the tube has a diameter of twenty-one mm. The elements 

 have not been observed in its passage through the air-chambers. 



The test baa a thickness of .75 mm. Surface marked by fine, irregular, 

 lamellose lines of growth. 



The internal mould presents a very smooth surface, and, in the specimens 

 from the soft shales, the sutures are indicated by a mere line. 



A fragment retaining its normal form has a length of seventy-five mm., of 

 which forty-five mm. pertains to the chamber of habitation. A compressed 

 chambered fragment, with a length of eighty-five mm., shows twenty-eight 

 air-chambers. 



This species somewhat resembles O. subulatum and 0. constridum, of the Ham- 

 ilton group; but it differs conspicuously in its excentric siphuncle and the 

 character of the constriction on the chamber of habitation. Other species, 



