22 



may not be accessible to some of the readers of. these pages, the salient 

 features of both will be briefl}' ])articularizecl. 



General characters. — Shell discoidal, compressed, peripheiy narrowly- 

 rounded ; umbilicus rather small, step-shaped and rectangular : surface 

 marked either by flexuous, transverse striffi, or by rib-like folds. 



1. Presumed typical form. — Shell discoidal, compressed, but not very 

 thin, the maximum width of the outer whorl being only one third less 

 than its height. Volutions four, the inner ones flat, smooth and neai'ly 

 two-thirds covered by those which succeed them. Outer whorl with 

 somewhat flattened but slightly convex sides, the greatest thickness 

 being a liitle below the middle; periphery rounded : inner edge of the 

 volution cut squarely at a right angle to the sides ; umbilicus step-shaped, 

 rather less than one-fourth of the entire diameter ; aperture elliptical or 

 narrowly oval, truncate below, higher than wide, deeply emarginate by 

 the preceding volution. 



Surface ornamented by faint, flexuous, and slightly raised, transversely 

 radiating stripe, which are usually arranged in two sets. What may be 

 conveniently called the primary stria; start from the umbilical margin, 

 and pass completely over the sides, but become indistinct or obsolete on 

 the periphery. Commencing at the umbilicus, their general direction is 

 first a little forwards, then backwards, and again forwards, until at last 

 the}^ form moderately convex but very faint arches over the pei'ij^hciy. 

 Near their junction with the umbilicus they are often seen to be made 

 up of two or even three separate raised lines, which coalesce about the 

 middle of the sides, between which point and the periphery they always 

 become most prominent, fold-like and distant. The secondary stria?, 

 which usually alternate with the primaries, radiate from the umbilicus, 

 but extend onlj^ half way across the sides. There is generally a single 

 secondary striation between eacli pair of primaries, but there are some- 

 times two, and at others none at all. Besides these markings, there 

 are numerous faint and short stria' on the peripheiy, but these dis- 

 appear before reaching the sides. Tlie strise on the siphonal edge 

 are much finer even than the secondaries, and seem to be distinct from 

 them. 



G-reatest diameter of the shell, four inches and two lines: width of 

 umbilicus about one inch. Width of aperture (which is identical with 

 the maximum of thickness) nearly fifteen lines: height of do., outside 

 of the emargination caused by the preceding whorl, twentj" -three lines: 

 depth of the emargination, six and-a-balf lines. 



The above description and measurements refer exclusivel}' to the largest 



