﻿SONNINIA BREVISP1NATA. 343 



In the same way, from those species in which the costate stage commences as 

 early, marginata, &c, the almost disproportionate size of the spines in regard to 

 the costa3 distinguishes magnispinata, not to mention the reclining ribs. From 

 ptycta, the much larger and more regular spines, and, in the costate stage, the 

 flatter-sided whorl with distinct inner margin, are the chief points of difference. 



Bonn, magnispinata is a rare form from Bradford Abbas and Halfway House. 

 In PI. LXXVI, figs. 1 — 3, I have fully illustrated the typical example ; and an 

 older specimen to show the costate stage is drawn in figs. 4, -5 of the same plate ; 

 its suture-line, fig. 6. 



Sonninia semispinata, 8. Buckman. Plate LXXVII, figs. 1, 2. 



Discoidal, compressed, carinate. Whorls, in section, oblong, ornamented with 

 regular spines, then with spines and reclining ribs, later with direct, reclining, 

 ventrally-inclined ribs, which tend to become more upright and obscure with age. 

 Ventral area somewhat angular, divided by a small carina. Inner margin well 

 marked, nearly upright, flat. Inclusion about three-eighths. 



The general shape of this fossil and its reclining ribs suggest that it is the 

 descendant of magnispinata, though there are certainly some links missing. On 

 the other hand, it lacks the strong keel of that species, though this might be a 

 feature correlated with the development. It differs from magnispinata altogether in 

 reduction of ornament and the earlier appearance of the costate stage. It is less 

 spinous, flatter-whorled, and more costate than ptycta, more spinous and flatter- 

 whorled than cymatera ; and, besides, its costee become smaller with age, but those 

 of cymatera larger. From spinifera it differs by being flatter-whorled, more 

 angular, and in having a pronounced inner margin. 



From morphological equivalents in the dominans- and other stocks, the reclining 

 ribs and the twisted appearance of the umbilicus will be found the chief points of 

 separation, although there are also differences in shape of whorls, costation, &c. 



This is a rare form from the Goncavum-zone of Bradford Abbas. It is 

 illustrated in PI. LXXVII, figs. 1, 2. 



Sonninia brevispinata, S. Buckman. Plate LXXV, figs. 6 — 8. 



Discoidal, compressed, carinate. Whorls in section oblong, ornamented first 

 with regular spines, later with spines parted by reclining ribs, 1 still later with 

 direct reclining ribs which presently become more upright. Ventral area narrow, 



1 To about 27 mm. diameter. 



