1870.] TATE— GLOUCESTERSHIRE LIAS. 401 
a change in the life aspects. And though this may be 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 
deeadence being attained in the zones of Ammonites owynotus and 
A. raricostatus, whilst from the zone of Ammonites Jamesont the 
species increase in number. 
A third phenomenon, but of minor importance, is that of the in- 
troduction of new genera on passing the upper limit of the Lower 
lias. I would cite Xiphoteuthis, Exelissa, Rotella, Opis, Isocardia, 
Gresslya, Megerlea, Waldhewmia (withafew 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 paleontological and stratigra- 
phical unconformity between the two formations to which cach zone 
respectively belongs. 
Part IJ.—Dzscrirrions or New Spectres. 
A.—From the upper part of the Lower Lias. 
PLEUROTOMARIA RARICOSTATI, noy. 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 strie ; the fissural band on the upper third of the 
lower part of the penultimate whorl, above which there is one en- 
circling thread and below four larger and several smaller and inter- 
mediate 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. 
