• 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 oil 
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 4J mm. where the tube 
has a diameter of fifteen mm., and a depth of seven mm. 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. 
