﻿SONNINIA SUBMARGINATA. 



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species — which may be known as the dominans-stock — are separable one from 

 another by very distinct characters ; for the six species belong to three distinct 

 stages of ornamentation, thus : 



Spinous. Costate. More or less smooth. 



crassisp in a ta, irregularis, marginata, dominans, modesta, simplex. 



The species which I am now going to describe are quite as easily separable 

 one from another in direct genetic line ; but the matter is not so easy in regard 

 to their respective relations to the forms of the dominans-stock. It would seem as 

 if the subseries took its rise from marginata, and produced homoplastic forms 

 analogous to dominans, modesta, simplex. These forms may, however, be distin- 

 guished by the following characters : 



In proportion to the development attained, i.e. to the degree of retrogression, the 

 forms of the submarginata-sm'es are thinner and, more umbilicate than are their 

 morphological equivalents in the dominans-sfodc. 



The relative thickness is a character more noticeable in handling the specimens 

 than in the most carefully-drawn plates ; but the persistently larger size of the 

 umbilicus will be brought out, and is quite sufficient distinction. 



Sonninia submarginata, 8. Buckman. Plate LXXI, figs. 1 — 3. 



Discoidal, compressed, hollow-carinate. Whorls, in section, oblong. The 

 sides in the costate stage much flattened. The ornamentation begins with regular 

 spines, changing, at about 20 mm. diameter, to ribs and occasional spines, and then 

 the numerous, rather small, direct, upright, ventrally-inclined ribs of the costate 

 stage are assumed early in life (at about 40 mm. diameter). Ventral area arched, 

 divided by a distinct, rounded, hollow carina. Inner margin well-marked, smooth, 

 upright, flat. Inclusion about one-third. Umbilicus open, rather flattened ; the 

 centre deeper and spinous. 



This form is distinguished from marginata by its flatter whorls more finely 

 ribbed. It is nearest to dominans y, Plate LXIX, but may be known by its wider 

 umbilicus, slower coiling, and less pronounced spines ending earlier. 



It comes from Bradford Abbas, but is rare. A portion of a very well-preserved 

 specimen is depicted in fig. 1, Plate LXXI ; the outline of its aperture is given in 

 fig. 2 of the same plate ; and its suture-line in fig. 3. The drawings are all of 

 natural size. 



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