﻿SONNINIA SPINICOSTATA. 



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considerable agreement with the large marginata ; for the superior lateral lobe is 

 wide- stemmed, and its lateral lobules are not greatly developed. 



On the other hand, there is a form which it is necessary to consider in this 

 connection : I have called it spinicostata. It is somewhat remarkable for being 

 so umbilicate and yet so much compressed ; but it is most noticeable for 

 possessing, in conjunction with these characters, spines for so long a period. 

 Omitting the spines, its general shape seems to bring it into line with the 

 rostatt i -forms ; it has the peculiar four-sided, somewhat angular whorl which 

 distinguishes the series. If it be the ancestor of costata, examples of several inter- 

 mediate stages must be missing ; because the spines, which remain in spinicostata 

 from youth to adult, are a very youthful feature of cost at a. If it be not the 

 ancestor of costata, it must be presumed that no descendants of spinicostata have 

 yet been found. 



If spivieoxfata be included in the costata-stock, it might be said that the series 

 can be distinguished from those hitherto described by being thinner and more 

 umbilicate, the compression of course becoming more pronounced and the 

 umbilicus smaller in proportion to development. There is another feature notice- 

 able in the higher forms : the narrow ventral area is rather flat and angular, and, 

 considering the retrogression, shows the vestiges of furrows rather plainly. 



Sonninia spinicostata, 8. Buchman. Plate LXXIII, figs. 4—6. 



Discoidal, compressed, carinate. Whorls, in section, nearly quadrate, orna- 

 mented with direct, somewhat reclining, 1 ventrally-inclined costre, of which in the 

 inner whorls about every other one is decked with a fairly prominent lateral 

 spine, and in the outer whorls nearly every rib is, in the middle, drawn up into 

 an obtuse knob. 2 Ventral area obtusely arched, divided by a small (hollow ?) 

 carina. Inner margin fairly defined, steep. Inclusion about one-fourth. 

 Umbilicus shallow and open. 



The preservation of the specimen leaves something to be desired — the central 

 whorls are wanting, and the outer whorls are somewhat damaged in places. 



The specimen was evidently spinous to within about three inches of its 

 aperture. In the inner whorls the spines are larger, but irregularly placed ; in the 

 outer whorls smaller, but placed on nearly every rib. 



The species differs from all the forms I have described, except irregularis, 

 acanthodes, and crassispinata, by the persistence of the spines. 3 From irregularis 



1 Towards the last, upright 



* The body-chainber is without test, but the knobs on the core indicate that the test was 

 evidently spinous. 



3 See note in Explanation of Plate. 



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