158 Memoirs of the Indian Museum. [Vol. VII, 



larger dimensions, the primary ribs may increase to as many as sixteen. The 

 increase is due to the appearance, one at a time, of an additional rib at the anterior 

 limit of the convexity of the base, just along the edge of the terminal zone of accre- 

 tions. The ribs are ribbon-like, and become more crowded and somewhat narrower 

 towards the anterior limit of the base, in consequence of which the intervals, which 

 throughout the greater part of the body-whorl are broader than the ribs, become, 

 towards the anterior extremity, of about the same width as the ribs or narrower. 

 As in the case of the spire, the ribs carry delicate spiral lines. In the great majority 

 of specimens the intervals are quite without any spiral ornaments, except usually 

 the most posterior and broadest interval, which is generally bisected by a thin 

 remnant of the median subsidiary thread continued from the spire, though there 

 are specimens in which even this last remnant has disappeared. In the case of 

 very young specimens of less than 30 mm. in height, every interval throughout 

 the body- whorl is bisected by a very thin intercalary thread. 1 With the usual 

 exception, as already noticed, of a feeble remnant in the most posterior interval, 

 all these subsidiary threads disappear before the shell has reached a height of 

 30 mm. Amongst the series of specimens in the Calcutta collection, the sculpture 

 of the body- whorl remains perfectly unaltered up to a total height of 100 mm. 

 It is only in quite adult specimens of still larger size that the sculpture enters 

 upon a new phase through the re-appearance of intercalary ribs, which may broaden 

 until they fill almost the whole of the available interstitial space, and assume 

 an appearance almost identical with that of the original primary ribs. On the later 

 part of the body-whorl of the largest specimen in the Calcutta collection, the seven 

 first intervals, counting from the posterior edge of the body-whorl, each carry an 

 intercalary rib. The eighth, ninth, and tenth are plain, but an intercalary rib also 

 appears in the eleventh. In the large individual illustrated in Reeve's monograph 

 (fig. 3a), some of the intervals carry two subsidiary ribs. The scarcely bulging, 

 torose, steeply winding zone of accretions is posteriorly bordered by a very thin, 

 narrow, sharply defined ridge. It is divided into two sub-equal portions, a posterior 

 one corresponding with the accretions of the deep indentation of the notch, across 

 which the lines of growth are deeply concave, and an anterior portion corresponding 

 with the accretions of the anterior border of the notch and anterior termination of 

 the shell, across which the lines of growth are convex. This anterior portion carries 

 three or four spiral ribs. The band corresponding with the accretions to the notch 

 may also carry spiral ribs, especially in the case of small specimens, but they are less 

 prominent than those of the anterior sub-zone, and, in many instances, are represen- 

 ted merely by some obscure distant spiral lines. The anterior termination of the 

 anterior sub-zone, forming the anterior termination of the shell, when well preserved, 

 is foliaceous and somewhat palmately expanded. The thin lines of growth, over the 

 greater part of the body- whorl, are strongly oblique and anteriorly antecurrent, and 



1 Martin has figured a fossil specimen from the pliocene beds of Java (Samml. des geol. Reichs-Museum in Leiden, 

 new series, Vol. I, pi. xxv, fig. 372), referred to DoNum costatum, and measuring about 30 mm. in height, in which the 

 intercalary median thread is faintly visible in several of the intervals of the body-whorl. 



