. CEPHALOPODA. 437 



over the chamber of habitation in the largest individuals, where the distance 

 may be two, three, or four mm., or even more; as indicated on the specimen, 

 pi. 67, toward the periphery. These striae, on the lateral faces, make a 

 eurvature very nearly corresponding to the curvature of the septa ; and in the 

 young shells they make an abrupt retral curve over the low saddle, thence 

 to the middle of the periphery, where they are recurved ; forming a deep 

 sinus upon the ventrum. The place of the saddle is indicated by a double 

 revolving groove, margined on each side by a low carina, and separated by 

 a stronger similar one, As the shell enlarges, the lateral of these two 

 grooves first becomes obsolete, and finally also the other, leaving a simple 

 obtuse angle along the line of the saddle, over which the striae in their retral 

 curve are more conspicuous. The form of the sinus of the aperture is indi- 

 cated by the direction of the striae on the ventrum. A single individual of 

 the species shows a smaller sinus on each side of the centre, and thence a 

 broad, shallow sinuosity reaching to the baso-lateral angles of the aperture. 



The internal cast, in the young specimens, often preserves the double 

 ventro-lateral, revolving grooves", or frequently a single groove; and also the 

 curving annular ridges upon the lateral faces. These are sometimes very 

 conspicuous, gradually becoming obsolete on the chamber of habitation, as the 

 shell increases in size. In the advancing growth of the shell the sides of the 

 outer volution gradually become flattened, and often a little concave toward 

 the outer margin, along a line parallel to the periphery. This feature is 

 more conspicuous in old shells, and is especially marked in the specimen on 

 plate G8. With these exceptions the cast of the interior is smooth. The 

 suture lines are strongly impressed, and the septa are imbricating, leaving 

 the posterior margin of the air-chamber curved or sloping, while the anterior 

 edge is angular. A slender 'median ridge or carina has been observed .along 

 the centre of the ventrum, upon a partially exfoliated specimen, marking the 

 inner or nacreous layer of the shell. 



The individuals of this species present a great range in size and proportions. 

 The smaller specimens do not exceed thirty mm. in their greatest dimensions, 

 witli the dorso-ventral and lateral diameters of their volutions about as ten 



