W. H. Hudleston — Gasteropoda of the F or tland Rocks. 391 



and semilunar, with a thick callus on the columella : considerable 

 umbilical groove. 



Relations and Distribution. — As before stated, this shell has been 

 regarded in its relations with Natica ceres, which is said to be 

 common in the Upper Portlandian of Wimereux. It is, however, 

 less globular, has a smaller spiral angle, is more rugose, and would 

 seem to attain to a larger size. Natica ceres, according to our 

 present knowledge, is confined in this country to a limited horizon 

 in the "creamy limestones" of Bucks, whilst the species under 

 consideration has a wider range. In an opposite direction its 

 affinities with such a form as Fs. rtigosa have been already indi- 

 cated, yet it is decidedly more Naticoid, and, although differing 

 somewhat from the typical Naticcs, may be regarded as belonging 

 to a section of that genus exhibiting peculiar modifications. There 

 is a form, for instance, in tbe Lower Lias of the East of France, 

 figured and described by Piette (Terq. and Piette, p. 31, pi. i. fig. 

 15-16) as Natica plicata, which, as far as the figure goes, possesses 

 considerable resemblance to Natica incisa. It is difficult to ascertain 

 how nearly the two are related, since the description of N. plicata 

 would seem not to coincide exactly with the figure : yet it is evident 

 that Natica plicata of the Lower Lias is a member of this section of 

 the genus. 



Natica incisa is pretty widely distributed on a certain horizon in 

 the " creamy limestones " of Bucks, and is tolerably numerous in the 

 Cyrena-heds of the Vale of Wardour, where, perhaps, the average 

 size of the specimens is greater than in Bucks. Mr. Blake quotes 

 it from the Portland Sand of Portland, where it would seem to be 

 a little out of place, as there is no record of its occurrence in the 

 limestones except those belonging to the Ctjrena type. Something 

 like it occurs in the Eoche, but the specimen is too imperfect to 

 be referred to N. incisa with certainty. 



5. Natica elegans, Sowerby, 1835. Plate XL Figs. 8a, 8b. 



Natica elegans, Sowerby, 1835, in Fitton, Trans. Geol. Soc. 2nd ser. vol. iv. p. 347, 

 pi. 23, fig. 3. 



This well-known species is merely introduced for purpose of 

 comparison with the other forms. It possesses a somewhat higher 

 spire (angle about 84°) than Natica incisa, and the median keel of 

 the body-whorl is scarcely perceptible. The posterior border of the 

 whorls is markedly tabulate without being indented : the contour 

 of the shell is very regular, and, as its name implies, elegant : the 

 ornaments consist of fine lines of growth often exquisitely preserved. 



N. elegans is very abundant in the " creamy limestones " of Bucks, 

 where specimens from 10 to 70 millimetres in height may be 

 obtained, showing the same character notwithstanding such differ- 

 ences in size. I have never seen any other than small specimens 

 from the Q/rena-beds of the Vale of Wardour, but casts of Natica 

 elegans of considerable size occur in some of the Portlandian beds of 

 that locality. It is one of those forms which seem common to both 

 types of Portland limestone. 



