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



this particular interval is of about the same thickness as the true median thread of the 

 second order and is shifted quite close to it, so that they both together form a 

 conspicuous pair occupying approximately the middle of the said interval, separated 

 from one another at first by a minute thread of the fourth order. The two com- 

 ponents of the pair gradually thicken and finally coalesce into one broad flat band, 

 which remains somewhat bifid and is considerably wider than either of the two other 

 main ribs (PI. V, fig. 5). Nevertheless, throughout the numerous specimens that have 

 been studied, the characters of the spire remain remarkably constant. The lines of 

 growth are straight and strongly oblique, antecurrent to the posterior suture, retro- 

 current to the anterior suture. At the earliest stages of growth they form an ex- 

 tremely delicate web, intersecting the spiral ornaments, the crowded thin raised lines, 

 much thinner than the three first orders of spiral ornaments, being distributed with 

 the utmost regularity. With increasing growth, the lines become relatively less 

 prominent and much less regularly distributed. 



The large body-whorl constitutes the greater part of the shell. It is globose, 

 almost spherical, exhibiting, on the right side of the shell, a continuous convex cur- 

 vature which, on the left side, is interrupted by the zone of accretions of the 

 deep terminal notch. Viewed dorsally, the zone of accretions is almost vertical 

 at its rather abrupt junction with the anterior flattened termination of the basal 

 convexity, and then assumes a convex outline becoming gradually more oblique in 

 an anterior direction towards the right of the shell. The zone of accretions winds 

 very steeply and bulges very feebly, which partly accounts for the narrowness of the 

 umbilicus. Including the ornaments continued from the spire, and omitting the 

 narrow ridge which posteriorly limits the terminal zone of accretions, the body- 

 whorl carries ten or eleven primary spiral ribs. They are broad and ribbon-like 

 though slightly convex. The two most posterior ribs, that is the circumsutural one 

 and the one continued from the angulation of the spire, are narrower than the 

 succeeding ones. The surface of the primary ribs frequently carries a variable 

 number of fine raised spiral striations. Throughout the greater part of the shell 

 the intervening spaces are much wider than the ribs, the two intervals continued 

 from the spire, especially the most posterior one of all, being generally broader than 

 the remainder. Towards the anterior termination, the primary ribs become more 

 crowded and at the same time narrower, though, as the diminution in size does not 

 exactly keep pace with the contraction of spacing, the three or four last intervals are 

 of about the same width as the adjacent ribs or only slightly broader. The number 

 of primary ribs remains exactly the same at all stages of growth : it is the same 

 in small specimens of less than 30 millimetres in height as well as in full-grown 

 shells of over ten centimetres. In those shells in which the spire develops an addi- 

 tional large rib by the coalescence of two subsidiary threads, the full-grown shell 

 may apparently exhibit as many as twelve main ribs, but the supernumerary rib 

 betrays its adventitious origin by its relative flatness, as well as by the disposition 

 of the subsidiary threads in the two adjacent intervals in which they are fewer than 

 in the true primary intervals, and lastly by the absence of the characteristic macula- 



