﻿SONNINIA DOMINANS. 



323 



central whorls only bear a few irregularly placed spines. Ventral area rather 

 broad and flat, carina hollow, 1 distinct but not very conspicuous, bordered by 

 depressed zones. Inner margin well-marked, upright, slightly convex. Inclusion 

 about one-third. Umbilicus becoming excentric 2 in very large examples. Suture- 

 line with superior lateral lobe tripartite, the outer lobule rather more developed 

 than the inner one. 



This species is exactly the next descendant of Sonninia marginata. The spines 

 do not end very much earlier, about half a whorl generally ; but there is in the 

 following stage a reduction of the ribbing correlating with a slightly smaller 

 umbilicus. This may be regarded as the further retrogression of the early costate 

 stage of marginata. In the later costate stage of marginata there appeared to be 

 progression, the ribs becoming larger and more distant ; and the same feature is 

 seen in dominans commencing at an earlier age. 



Of this species there may be said to be certain varieties which no doubt owe 

 their origin, really, to less-marked forms of ma rginata, of which forms they have 

 accentuated the differences sufficiently to be noticeable. Among the chief forms 

 there are — 



a. The type, PI. LXVI. 



/3. The umbilicate form. 



y. The close-ribbed form, PI. LXIX. 



B. The thick-whorled, wide-ribbed form. 



The last two are the more-ribbed, the former two the less-ribbed forms. At 

 least two distinct sub-series take their rise from the above varieties, and probably 

 more may yet be detected by careful work. 



Bonn, dominans is one of the commonest of the Concav inn-zone species of the 

 genus. Its characters are fairly well-marked ; and, though it can easily be 

 connected by intermediate forms with marginata, it is not difficult to recognise 

 it from that species on account of the general reduction in ornament. 



It is also the largest species of the genus in the Concavitm-zoiie, its only rival 

 being the large marginata figured. I have in my collection three large specimens 

 of dominans, which illustrate three different phases of development. The first is 

 a rather costate form which, roughly speaking, may be said to connect the large 

 marginata with the large type dominans. It is 318 mm. in diameter, thus surpassing 

 marginata. It possesses what appears to be the mouth, 8 the test being much 



1 Iu this and other species crystallisation often obscures the structure of the carina, so that a 

 hollow carina may easily be overlooked. 



2 Umbilicus excentric : — -the body-chamber becomes narrower, causing the inner margin to recede 

 from the normal line of coil, and so the umbilicus is suddenly widened. 



Umbilicus concentric : — when a normal and regularly-increasing mode of coiling is maintained. 



3 A broken specimen in my collection, about 2G0 mm. (10^ inches) in diameter, shows an 

 unmistakable mouth possessing good test on the ventral lappet. The mouth is quite plain, appears 



