239 



(plate 3, fig. 1,) the inner whorls are partly absent, and in part concealed; 

 iu SowERBT's type, only four whorls can be counted, the central ones 

 being absent; in d'ORBiGNY's fine specimen, which is almost perfect, 

 there are eight whorls, and this we may regard as the normal number 

 in a shell 180 millemetres in diameter; the number of ribs in a whorl 

 differ much in difi-ereiit specimens, thus in Sowerby and Zieten-s shells 

 there are 27; in Hauer's, 34; and in d'ORBiGNv's, 45; they are simple, 

 strong, and arched, and terminate near the back in a large, round, 

 obtuse tubercle. Sowerby says, "the ribs rising in knobs just as they 

 reach the front give the whorls a very square aspect; the ribs and the 

 hollows between them are nearly equal. The back is wide, in consequence 

 of the extension given to this region by the size of the costal tubercles; 

 upon the middle line of the back is a strong, thick, rounded keel, having 

 on each side thereof a deep sulcus, bounded by lateral elevations, so that 

 the broad back is tricarinated with two intei-vening sulci. The spire is 

 composed of quadrate whoris, with prominent costse, and a series of 

 nodules along the cur^-ature of the spire. The aperture is quadrate, 

 depressed, and sinuous anteriorly. 



The septse in this species, according to M. A. d'ORBiGNY, are sym- 

 metrical, divided on each side into four lobes, of which the two external 

 lateral are formed of unequal parts. The dorsal lobe, (plate 3, fig. 1«.) 

 one-third longer than the superior lateral lobe, is ornamented on each 

 side with a long denticulated branch. The dorsal saddle, one-half the 

 width of the superior lateral lobe, formed of three divided leaves, of 

 unequal size, of which the median is the largest. The sv^erior-lateral 

 hie terminates by two points, and has on each side three or four others.^ 

 The lateral saddle, narrower than the superior-lateral lobe, is foi-med of 

 three unequal leaves, resembling those of the dorsal lobe. The inferior- 

 lateral lobe, half the width of the superior-lateral lobe, tenninates by two 

 points. The two auxiliary/ lobes are very small, and divide into one 

 median and two lateral points. The central ray, in parting from the 

 extremity of the dorsal lobe, passes well below all the lobes externally 

 visible, but reaches the extreme ventral lobe, which is bifurcated, and 

 accompanied on each side by a very large ventral saddle." 



Affinities and Differences.— This species resembles in many characters 

 A. Conybeari. It is distinguished, however, from that form, by having 

 a wider back, fewer vibs, and each terminating in a large obtuse tubercle; 

 the septse are likewise much more complicated than those of A. Conybeari. 

 It resembles A. Bisulcatus, Bi-ug., in possessing simple ribs with dorsal 

 tubercles, and two deep sulci on each side of the median keel. It is 



