﻿SONNINIA MODESTA. 



325 



marginata. The inheritance of this elaboration appears to be affected by the 

 preceding reduction. It is not the mere size of ribs that must be considered, but 

 the amount of contrast between the effects of the reduction and the effects of the 

 elaboration. It may be seen that in revirescens the contrast between reduction 

 and elaboration, or between the retrogression and progression, is greater than in 

 marginata or dominans, and that it is manifest earlier. 1 



It can scarcely be doubted that Sonn. revirescens is a development of the type 

 of Sonn. dominans, for, as regards its earlier whorls, the same changes in relation 

 to dominans are continued as distinguish dominans in relation to marginata. 

 In its outer whorls the coarse distant ribs attain a larger size. The renewal of 

 progression, so far as the ribs are concerned, has induced me to distinguish the 

 form by the name " revirescens. " It is a very rare form — this is the only large 

 example in my collection. It came from Bradford Abbas. 



Fig. 1 of PI. LXX illustrates a portion of the specimen reduced to one-half 

 the natural size. The drawing is a copy of a photograph showing the essential 

 features — the change in ribbing and the umbilicus. In comparing this figure 

 with that of the large dominans in PI. LXVI, it must be remembered that both 

 are reduced — this one the most. 



Sonninia modesta, 8. Buckman. Plates LXVIII, LXX, fig. 5. 



Discoidal, compressed, hollow-carinate. Whorls, for the most part, smooth 

 or feebly ribbed — only the central part of the umbilicus bearing a few rudimentary 

 spines irregularly placed on small ribs. Ventral area rounded, divided by a small 

 but well-marked, and cleanly-separated, hollow carina. 2 Inner margin broad and 

 conspicuous, upright, flat. Inclusion in youth one-half, decreasing to about one- 

 third in older specimens. Umbilicus regularly graduated — becoming excentric 

 with age. Suture-line of the general Sounini a -type, but the outer lobule of the 

 superior lateral lobe rather excessively developed on account of the greater breadth 

 of the whorls. 



The specimen depicted in PI. LXVIII is not a typical form of the species, 

 because it exhibits a peculiar character not observable in the other examples to 

 anything like this extent, even when they are much older. I refer to the ribs of the 

 last quarter-whorl, which are almost tubercled at their middles. These ribs might 

 have been produced by a process of development similar to that which elaborated 

 the enlarged ribs in senile dominans and in revirescens; and in that case I should 



1 Compare remarks on umbilicus, p. 327. 



2 The carina loses the " hollow " character with age. 



