LIOCERAS CONCAVUM. 57 



have had figured again so as to show its true characters accurately. Closely allied 

 to the typical Lioc. concavum, and occurring in the same bed with it, we find a large 

 number of different forms certainly capable of separation (pp. 72 — 76), although it 

 requires careful and accurate observation to effect it For such a purpose 

 Sowerby's figure in the ' Mineral Conchology ' was not sufficient. These different 

 forms of Lioc. concavum may be most conveniently classed in two groups as 

 follows : — The varieties in the first group develop at a very early age sigmoidal 

 radii, which are projected forwards on the ventral area; those in the second 

 continue for a long time (even to attaining the size at which the former 

 become nearly adult) to possess only the V-shaped radii. Such ribs or radii 

 take the form of an expanded <!, with its apex pointing forward on the lateral 

 area, while its outer end meets the ventral area and passes over it and over the 

 carina 1 nearly at right angles. (Specimens with these ribs are particularly shown 

 in PL IX, figs. 6, 7, and PI. X, figs. 5, 7.) It is only at an advanced stage that 

 the V-shaped radii give place to sigmoidal curves. The varieties composing the 

 second class are also, generally speaking, coarser in their ornamentation, of larger 

 growth, with less compression, and a smaller carina. 



If we compare all the forms which I shall include under " Lioceras concavum and 

 varieties," we observe that there is a very general similarity among them. The 

 inner area devoid of ribs and made subconcave by the edge of the inner margin being 

 raised, the nearly parallel sides, the sloping ventral area, and lastly the " concave " 

 umbilicus with traces of ribs in its inmost whorls, are the most persistent characters. 

 Although the whorls are not always occluded quite to the edge of the inner margin, 

 yet the slope of this inner margin, together with the less occluded whorls of youth, 

 always give to the umbilicus a definite character, namely, its peculiarly excavated 

 appearance. 



By breaking up a specimen to obtain the inner whorls, we are able to find out the 

 manner in which the concave umbilicus has been formed. Until about the third 

 whorl (diameter about 2 lines) the aperture is almost round — being about as broad 

 as it is long — while the whorls are not at all compressed. This third whorl is 

 occluded about one half by the next, which has its sides much flattened. The 

 breadth of its aperture is one-and-a-half times that of the corresponding 

 portion of the previous whorl ; while its length has increased to two and a quarter 

 times. This whorl is about three-fifths occluded by what is about the fifth whorl ; 

 and in this fifth whorl we meet with the commencement of the concave inner 



1 The radii on the ventral area are only striae. The term " radii " seems to be convenient in 

 describing that combination and alternation of ribs and stria? which make the ornamental curves on 

 the shell from the inner to the outer margin. When thus understood the term obviates the necessity 

 of an explanation each time a change (whether from ribs to stria? or vice versa) takes place along one 

 of the curves. 



