Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/567

1870.] TATE — GLOUCESTERSHIRE LIAS. 401 a change in the life aspects. And though this may he attributable to certain physical changes, it does not necessarily imply the advent of a new period, the change of deposit and life representing often different regions of depth ; instances of such mutability are not wanting in the Lias. But in the case of the upper part of the Lower Lias the conditions of depth and of deposit are repeated in the lower part of the Middle Lias, accompanied by a total change in the fauna, and not a recurrence, as we should expect if there had been an uninterrupted continuance of the conditions by which these clays had been accumulated.

A second phenomenon that gives support to the views expressed above, is that of the numerical decrease of specific forms as we pass up through the several stages of the Lower Lias, the maximum of decadence being attained in the zones of Ammonites oxynotus and A. raricostatus, whilst from the zone of Ammonites Jamesoni the species increase in number.

A third phenomenon, but of minor importance, is that of the introduction of new genera on passing the upper limit of the Lower Lias. I would cite Xiphoteuthis, Exelissa, Rotella, Opis, Isocardia, Gresslya, Megerlea, Waldheimia (with a few exceptions), Terebratulina, Acroura, Uraster, Tropidaster, Luidia, Plumaster, and Astropecten. The absence of corals is also to be noted.

In conclusion, the fact of the similarity in the lithological features of the zones of Ammonites raricostatus and Am. Jamesoni, with a very large percentage of distinct species, and the fact that the former horizon contains a fauna representing a marked decadence of Lower- Liassic species, and the latter zone a fauna consisting, in the greater part, of Middle-Liassic species, determine the attachment of the one to the Lower Lias, and of the other to the Middle Lias ; and show that the line of demarcation is not arbitrary, but is in accordance with a greater or less unconformity of specific forms, perhaps sufficiently great to warrant us in assuming a palaeontological and stratigraphical unconformity between the two formations to which each zone respectively belongs.

Part II. — Descriptions of New Species. A. — From the upper part of the Lower Lias.

Pleurotomaria raricostati, nov. spec. (Pl. XXVI. figs. 9 & 9a.)

Shell turbinated, broader than high; whorls subquadrate, with small nodulations upon their angles, crossed by four longitudinal slender raised lines ; upper surface of penultimate whorl with about six encircling equidistant raised threads, decussated by closely set and anteriorly curved striae ; the fissural band on the upper third of the lower part of the penultimate whorl, above which there is one encircling thread and below four larger and several smaller and intermediate longitudinal threads, the whole crossed by curved lines of growth ; base slightly convex, subimperforate.

Dimensions. Height 2 inches ; breadth 2-25 inches ; height of last whorl 0-9 inch.