﻿SONNINIA MARGINATA. 



3.2 L 



Both a and /3 are descendants of crassispinata (3, though |3 retains the 

 reclining style of ribbing, while in a the ribs become upright. The early 

 adolescent stage of a is the exact morphological representation of adult 

 crassispinata /3. 



The form y is evidently an involute development of crassispinata y. It is so 

 different from the others that it deserved delineation, but space does not allow 

 a figure. 



The "irregularity" of ornament is the chief distinction of irregularis. Its 

 more compressed, flatter-sided, more quickly-coiled whorls, with marked inner 

 margin, separate it from the foregoing species. Id addition, the more upright ribs 

 distinguish the form «, and the earlier cessation of the spines the form (5 from the 

 previous species. It is impossible to confound y with any of them, for in spite of 

 its spines it is much more retrogressive than any others. It is a marked retro- 

 gression of the least progressive form of crassispinata , namely y. 



Sonninia irregularis is commoner than the species hitherto described, and, by 

 an almost perfect series of gradations, I can connect it with the succeeding form. 

 It occurs in the Coneavum-zone of Bradford Abbas, Dorset. 



Taken in consideration with the remarks just made, the specimen figured in 

 PI. LXI represents the characters of the species. It is one of the largest 

 examples of the species in my possession, and shows in a marked degree the 

 "irregularity" which was a prelude to loss of spines. With a certain 

 allowance for the " unequal action of the law of earlier inheritance," the young- 

 stages of crassispinata (PL L, figs. 19 — 22) will illustrate the young forms 

 of this species. It may be noted that in this species any representation of the 

 first irregular spinous stage — that of crassispinata a — has been omitted from the 

 ontogeny. I should remark that the opening description (previous page) applies 

 to the type-form only. 



Sonninia marginata, 8. Buckman. Plates LX1I, LXIII, fig. 2 ; Plates LXIV, 



LXV, figs. 1, 2. 



Discoidal, compressed, hollow-carinate. Whorls compressed, sub-quad- 

 rangular, ornamented with direct, nearly upright, ventrally-inclined ribs, and, in 

 youth only, with spines, at first regularly on the ribs, afterwards missing some. 

 Ventral area rather broad, hollow-carinate ; the core slightly carinate, bordered 

 by slight depressions. Inclusion about one-third. 1 Inner margin well marked, 

 upright, slightly convex. 



Fine growth-lines run parallel to the ribs. In old age the ribs become larger 

 and further apart. 



1 See, however, footnote 2, p. 320. 



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