﻿SONNINIA PARVICOSTATA. 



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this species a peculiarly angular appearance. The irregular, well-marked ribs, the 

 absence of spines, 1 and the large umbilicus make the species very distinct from other 

 Sonniniae of the one avwm -zone. It has no morphological equivalent in either 

 the dominans- or submarginata-series, for in both of those genetic series the spines 

 persist until the ribs are much more reduced in size. In certain respects it may 

 be considered, a morphological equivalent of obtusiformis, because that is a wholly 

 costate, non-spinous form ; but the least comparison as to the shape of the aperture 

 and the size of the ribbing will show what great differences exist between the 

 two shells. 



Though not much like any other species of its own genus, Sonn. costata has 

 considerable resemblance to Dumortieria grammoceroides (page 262); but, besides 

 the differences in the suture-line — the inner portion less retracted, and yet with 

 larger lobes — which are of generic value, this species is more quickly coiled, less 

 umbilicate, and much more carinate. The typical form of costata is also nearly 

 twice as thick as Bum. grammoceroides ; but the forms between costata and 

 parvicostata are not very much thicker, though in the other points of difference 

 the separation is quite as marked. Considering that these species belong to two 

 different families, the points of superficial resemblance are very interesting. 2 



This is a common and well-known fossil at Bradford Abbas. It is one of the 

 characteristic species of the Concavum- zone ; and it attracts more than its due 

 share of attention, because it is so easily identified. In PI. LXXIV, fig. 1, is 

 shown (two-thirds natural size) a fine example of the form «, rather more costate 

 than in the usual run, perhaps. The outline of its aperture is delineated in fig. 1 

 of PI. LXXV ; and its suture-line, of natural size, in fig. 2 of the same plate. 



Sonnikia paevicostata, 8. Buckman. Plate LXXIV, figs. 2, 3 ; Plate LXXV, 



figs. 3 — 5. 



Discoidal, compressed, carinate. Whorls, in section, compressed, oblong, for 

 the most part smooth, but in infancy and early adolescence ornamented with direct, 

 upright, ventrally-inclined costee, which, later on, give place to some obscure 

 irregular bulgings. Ventral area rather flat and fairly defined, divided by a small, 



1 I do not mean to say that no rudimentary spines can be detected in exceptionally well-preserved 

 specimens — in fact, irregularly-placed spines may be seen in some examples to about 12 mm. diameter; 

 but in the ordinary run of by no means ill-preserved examples no spines are seen, and for all practical 

 purposes the species may be called non-spinous. I wish this l'emark to apply in other cases among 

 the Sonnininse. The spines, however rudimentary, are interesting as a proof of descent. 



2 Dumortieria did not reach a spinous stage. There are no spines in the inner whorls of 

 Dum. grammoceroides. See also description of ontoyeny, p. 265. 



