CEPHALOPODA. 477 



Volutions six or more, in entire individuals, all exposed in the very wide, 

 shallow umbilicus, a small proportion only (less than one-fifth) of the inner 

 being overlapped by the outer ones, which are grooved upon their inner 

 margins for the reception of the periphery of the inclosed volution ; the 

 groove has a depth of about two mm. where the dorso-ventral diameter of 

 the embracing volution is twenty-one mm. Transverse section of the outer 

 volutions trapezoidal (of the inner ones semi-elliptical), the sides nearly 

 flat, the apex truncated, with the angles rounded ; the base is concave in the 

 middle, with the baso-lateral margins rounded. The greatest transverse 

 diameter is equal to two-thirds of the dorso-ventral diameter, the difference 

 in the proportions increasing in the outer volutions. The volutions enlarge 

 very gradually ; the measurements give seven and thirteen, and eleven 

 and eighteen, or an increase of six and seven mm. in a single turn. In a 

 larger individual the increase in half a volution is from nineteen to twenty- 

 three mm., or equal to eight mm. in a single volution. 



Chamber of habitation unknown, all the specimens observed being incom- 

 plete. Aperture unknown. Air-chambers numerous, slightly irregular in 

 depth, and as a rule, gradually increasing toward the outer chamber. 



Septa strong, distinctly thickened and imbricating at their margins, and 

 remarkable for the regularity of their curvature. From their origin they 

 advance toward the aperture, in a nearly direct line, obliquely over the 

 umbilical margin to a point on the lateral face about one-fifth of its diameter; 

 thence bending abruptly backward they describe an extremely abrupt curve, 

 including a linguiform mucronate lobe, and return to a point in the centre of 

 the volution greatly in advance of their first retral bending. From the 

 summit of this curve or saddle on the centre of the face of the volution they 

 describe a somewhat longer and scarcely wider retral and advancing curve, 

 which in like manner limits a narrow linguiform and mucronate lobe, and 

 from which the septum curves over the margin of the periphery, thence 

 abruptly descending and turning acutely forward describes a narrow triangular 

 mucronate lobe on the ventral side. This order gives an obtusely subtrian- 

 gular saddle on the umbilical margin, a narrow lobe, and a very narrow and 



