CEPHALOPODA. 465 



Chamber of habitation very large, in one specimen occupying more than 

 half a volution ; its full extent not known ; its capacity is at least twice as 

 great as all the chambered portion of the shell. Aperture elongate, semi- 

 elliptical, narrowing toward the anterior margin, which issubacutely rounded. 

 Ba.se deeply indented by the preceding volution, and inferring from the form, 

 it is subauriculate at the baso-lateral angles. Air-chambers numerous, often 

 somewhat irregular in their depth, those near the chamber of habitation 

 sometimes shallower than those preceding. In a portion of the tube, which 

 has a lateral diameter of about forty mm. at the smaller extremity, 

 and of forty-six mm. at the base of the chamber of habitation, making 

 about one-quarter of a volution, there are nine air-chambers, which vary 

 in depth from six to ten mm., as measured from the summits of the lateral 

 .saddles, the shallowest being the last but one. In a smaller individual there 

 are nine chambers in less than half a volution of the septate portion pre- 

 ceding the commencement of the chamber of habitation. 



The septa are strong, especially in older shells ; much thickened on their 

 exterior margins, and strongly imbricating: In their course from their 

 origin on the umbilical margin they curve more or less abruptly backward 

 to a point from one-quarter to one-third the diameter of the volution, or 

 sometimes even less, where they make an acute return, and curving forward 

 over the lateral face of the volution, make a retral bend, which terminates at 

 a point within one-fifth or one-sixth of the- width of the volution from the 

 peripheral margin, whence they make an acute turn forward, and pass over 

 tli' 1 margin of the periphery in an abrupt curve, and descending slightly, 

 describe a narrow, acute lobe upon the centre of the ventrum. This course 

 of the septa includes a narrow acute lobe, near the inner margin of the volu- 

 tion, and thence describes a broad, obliquely semi-elliptical saddle; a second 

 narrow, elongate acute lobe, near the outer margin, and a narrow obtuse 

 saddle on the periphery, with one side extremely elongate, and the other 

 very short. The septum describing the wide lateral saddle extends forward, 

 in its advancing curve, to a distance equal to the greatest depth of nearly 

 two air-chambers beyond its origin at the umbilical margin. In its retral 

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