Whatever may be the precise biological value of this genus or subgenus as distinct from Purpura, it forms a remarkable group, limited, according to our present knowledge, both in time and space. Of the conditions favourable to its development we only obtain glimpses here and there. In the Coral Rag of Yorkshire one would say that the genus displays a remarkable tendency to variability within certain limits, so much so that almost every specimen found in anything like a decent state of preservation differs in some point of ornament: hence the extreme difficulty of specific arrangement. One might almost suppose that, like the Ammonite, the genus having worked out the limits of possible change, flourished within a short period only, and speedily became extinct.

Two papers, presented by Prof. Hughes to the Geological Society during the last twelve months, call for some comment from myself, as in many respects they controvert, directly or indirectly, opinions which I have expressed before the same Society. In the debates which followed I objected to the imperfect evidence on which these criticisms were founded; and as the author repudiated with some warmth my right to demand that conclusions founded not only on field work, but also on microscopic study, should not be assailed from the former point of view alone, I will venture to state the reasons why I so did, and intend so to do on any future occasion that may arise.

Sp. char.—Tube small, sinistral, narrow, increasing but little in size towards the aperture, very narrow and fine towards the apex; volutions not in the same plane, somewhat raised, with a sunken apex, rather deeply umbilicated on the attached side; section circular. Surface marked with faint, regular, equidistant, transverse ridges, which are more marked along the union of the whorls (or “suture”) than on the periphery or back of the last volution.

The following instance of the unconformability of the Keuper and Bunter divisions of the Trias, which came under my notice during the construction of the Great Northern Derbyshire Extension line in the year 1877, is deserving of notice, not only from its decided rarity, but also from its exceptional clearness.

This paper may be regarded as a notice of those portions of the recently published “Manual of the Geology of India,” by Messrs. Medlicott and Blanford, which describe, or in any way relate to, the extensive lateritic deposits of Peninsular India. The hypothesis (or, more properly, the modification of an antecedent hypothesis) herein offered, in the hope that it may be tested by the Indian geologists, is based on extended examinations of deposits believed to be analogous, and has been found to satisfactorily explain the phenomena observed.

Speculation having been rather freely applied to the identification of these rocks, I beg to offer a few notes upon the subject, the facts regarding which I have described in a paper already published.