CEPHALOPODA. 451 



to the chamber of habitation, which is more r*pidly expanding. Umbilicus 

 small, rounded and very neatly defined, with the margins subangular, and 

 the sides nearly vertical, the outer volution embracing almost the entire 

 width of the inner volution. Transverse section in young shells broadly 

 semi-elliptical or semicircular, with the basal or inner margin deeply indented 

 by the preceding volution ; the outer volutions and section of chamber of 

 habitation semi-elliptical, with the vertical much greater than the transverse 

 diameter ; the sides curving gently, the apex rounded, and the base deeply 

 indented ; the baso-lateral angles are slightly auriculate. The enlargement 

 in a single volution is from eleven to twenty-one mm. — the last measure- 

 ment being over the first half of the grand chamber; and in one-half a turn 

 of the last septate volution the increase is from eleven to fourteen. 



Chamber of habitation moderately large, gradually expanding and some- 

 what ventricose, occupying apparently the greater part of one volution. 

 The aperture is somewhat broadly semi-elliptical, with the sides gently 

 curved, the anterior margin abruptly rounded and deeply sinuate. The base 

 is deeply indented by the preceding volution, and the lateral angles slightly 

 auriculate. Air-chambers irregular, gradually expanding from the umbilical 

 margin, with the greatest depth usually upon the periphery; their depth 

 on the lateral face varies from 1.5 to 4 mm. in less than half a volution on 

 the same specimen, being equal to the greatest depth on the peripheral 

 margin- 



Septa thin, unequally distant at their origin, radiating from the umbilical 

 margin, and curving gently forward to the middle of the lateral face of the 

 volution, thence gently backward to a point about two- thirds the distance 

 from the centre of the umbilicus to the periphery, where they suddenly bend 

 backward, almost at right angles with their previous direction, to a point 

 below that of their origin, and in a line nearly parallel to the periphery; 

 following and partially producing the inner margin of a shallow, revolving 

 groove upon the surface ; thence bending gently forward to near the centre 

 of the periphery where they turn abruptly backward, defining the short, 

 narrow, ventral lobe, which penetrates a little more than one-third the depth 



