CEPHALOPODA. 439 



Volutions three or more, the outer ones rapidly expanding. Umbilicus 

 open, exposing all the turns of the spire; margins of the inner ones rounded, 

 and of the outer one angular, and bending almost rectangularly from the 

 lateral face. Inner volutions embraced by the succeeding ones to the depth 

 of one quarter or more of their dorso-ventral diameter. Transverse section 

 elongate-trapezoidal, with the sides very little curved ; the summit narrow 

 and gently curving, and the base indented by the preceding volution ; the 

 baso-lateral angles obtuse and slightly auriculate. The inner volutions 

 gradually increase in size at each turn, and are more rapidly expanding in 

 the outer ones. The rate of increase cannot be satisfactorily determined on 

 account of the condition of the specimens. The chambered portion of one 

 specimen shows an increased diameter, in a single volution, from eight to 

 twenty mm. 



Chamber of habitation large and deep ; its extent and proportions not 

 fully determined. Aperture, inferring from a section of the grand chamber, 

 elongate- trapezoidal, with the sides nearly straight, and the peripheral 

 margin narrow. Air-chambers regular, of moderate depth, gradually increas- 

 ing with the enlargement of the tube toward the chamber of habitation, and 

 on the middle of the lateral face, having a depth of 4£ mm. where the tube 

 has a diameter of fifteen mm., and a depth of seven jnm. near the chamber of 

 habitation, where the lateral diameter of the volution is twenty-six mm. 



The septa are thin, uniform, with the margins thickened, curving a little 

 forward on the umbilical margin, and thence making a gentle retral curve, 

 describing a broad simple lobe, which occupies almost the entire lateral 

 face of the volution, and advancing more abruptly toward the periphery, 

 make an acute retral, bend on the ventro-lateral margin, defining a subacute 

 saddle, and meeting upon the ventrum, include the elongate, acute ventral 

 lobe. The lateral lobe has a depth about equal to the depth of a single air- 

 chamber. 



Suture lines distinctly marked by the thickened margins of the septa, 

 which, on the broad lateral lobe, are gently imbricated toward the aperture, 

 the imbrication being more extremely marked at the acute curving over the 

 saddle. 



