﻿SONNINIA OBTUSIFORMIS. 



333 



costge, some of which, in the central part of the umbilicus, carry a small spine. 

 Ventral area broad, laterally-gibbous, somewhat flattened in the middle, with a 

 distinct, rounded, presumably hollow carina. Inner margin ill-defined. Inclusion 

 about one-fourth. 



This description is from the small example figured. In comparison with its 

 ancestor, ptycta, it is very distinct on account of the very small spines, which fail 

 altogether at a much earlier period. 1 In comparison with the next species, its 

 descendant (obtusiformis), the costas are much smaller — in other words, the larger 

 costse appear earlier in obtusiformis than in cymatera. This is confirmed by a 

 larger specimen in my collection, which, judging by its inner whorls, is exactly 

 intermediate between ptycta and cymatera ; it has spines larger than the latter, 

 but smaller than the former, while the ribs on its outer whorls (both core and 

 test) are decidedly more distant than in ptycta, and appear much later than in 

 obtusiformis. 



Sonn. cymatera is the morphological equivalent of dominans and of sub- 

 marginata, but differs from both in having larger, more reclining costas, and in 

 wanting a defined inner margin. Its inner whorls are much less elaborately 

 ornamented than those of marginata. The costre reclining at this rather advanced 

 stage of retrogression — shown by the rudimentary state of the spinous stage — 

 indicate that it belongs to the acanthodes-stock ; and the early appearance of the 

 costate stage reveals its position in the series. Further, in early adolescence it is 

 the morphological representation of adult ptycta, as the figures so plainly prove 

 (see p. 332). 



Sonn. cymatera is from the Concavum-zone of Bradford Abbas. A side-view of 

 a young example is shown in PI. LXXIII, fig. 2, the outline of its aperture in 

 fig. 3 ; both of natural size. 



Sonninia obtusiformis, 8. Buckman. Plate LXXII, figs. 3 — 5. 



Discoidal, compressed, hollow-carinate. Whorls, in section, gibbous-sided, 

 elliptical, ornamented with undulate, direct, reclining, ventrally-inclined ribs, 

 which become less reclining and considerably larger the older the shell, until they 

 resemble large waves. Ventral area broad, laterally-gibbous, the middle 

 depressed into a well-marked hollow, which is divided by a thick, rounded, hollow 

 carina, so that the area is carinate-bisulcate. Inner margin not actually defined, 



1 In comparing with ptycta it must be remembered that the figure of that fossil is an adult 

 reduced one-fourth, and that the spines and ornaments, which appear of about the same size as those 

 of the young cymatera, must be enlarged four times. 



