CEPHALOPODA. 
401 
ceding. The depth of the chambers is sometimes five mm. where the 
diameter of the tube is thirty-five mm., and from three to four mm. when 
the tube has a diameter of nineteen mm. 
The septa are thin, presenting, at their origin on the umbilical side, a 
gentle forward curve for the distance of a few millimetres, and are a little 
more abruptly recurved to about the same depth. From this point they 
make an abrupt angular return describing a broad curve over the middle of 
the lateral face, and descending a little deeper than before, they make an 
acute return and describe a shorter curve to the peripheral margin. By this 
disposition of the septa we have a narrow convex umbilical saddle, a broad 
central or lateral saddle, and a narrower peripheral saddle of a little greater 
width, but more convex than that on the umbilical side. It gives a short 
acute lobe, with its apex about one-fourth the width of the tube from the 
umbilical side, and a larger triangular lobe, with its apex at a point about 
one-fourth the width of the tube from the peripheral margin. The direc¬ 
tion of the septa on the periphery is undetermined. The sutures are usually 
but faintly impressed, and often very obscure upon the casts of the shell. 
Siphuncle unknown. 
Test strong, rarely preserved, having in old shells an approximate thick¬ 
ness of nearly two mm. on the chamber of habitation. Surface marked by 
strong sublamellose striae, which make a gentle retral curve over the lateral 
face of the volution, and arching somewhat abruptly forward as they approach 
the margin of the periphery are suddenly recurved over the ventral side. 
The striae are somewhat regular and even on the lateral faces of the volutions, 
there being often about six in the space of five mm^ and on the smaller 
volutions ten in the same space; approaching the aperture they become 
stronger, and there are only three in the space of five mm. The sinus', 
inferring from the direction of the striae, is abrupt and of moderate depth. 
The internal mould is essentially smooth, and but faintly marked by the 
suture lines. The external striae are sometimes impressed upon the cast, and 
in some conditions, the partial casts or interior of the shell, show a punctate- 
marking. 
