Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/566

400 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 23, include Avicula cygnipes, Waldheimia perforata, Cidaris Edwardsii, and some others ; but their number is not so great as is usually- supposed. The tendency among palaeontologists to name every smooth punctated Lima, L. punctata, every scalariform striato-nodulose Pleurotomaria, P. similis, or every planulate Cryptoenia, C. expansa, leaves no doubt that two species are often included under one denomination and that the numbers in common between the Lower and Middle Lias are doubtless fewer than is generally stated. I find, in the majority of cases, that the species cited from the Lower and Middle Lias belong to sectional groups, the species composing which have a general resemblance one to the other, and the differential characters cannot always be recognized at a glance. Of the many examples of the genus Cryptoenia from the Lower Lias that I have examined, not one is referable to C. expansa, which is quoted from that formation. It is possible that some species of the zone of A. raricostatus may have been mingled with those of the overlying stratum during the period of its deposition ; the similarity of the included matrices prevents a determination of the source of such presumed derivative fossils.

It may reasonably be urged that as the faunula of the zone of Ammonites Jamesoni forms an integral part of the fauna of the Middle Lias, and that as the species belonging to the zones of Ammonites raricostatus and A. oxynotus present nearly as great affinity to those constituting the fauna of the Middle Lias as they do to those of the Lower Lias, the line of demarcation between the Middle and Lower Lias should be drawn between the zones of Ammonites obtusus and A. oxynotus. This arrangement would give a total of one hundred and sixty-four species for the united zone, seventy- eight peculiar, fourteen from lower horizons, and sixty passing to higher beds of the Middle Lias. But many of the species which, in the Gloucestershire area, are restricted to the zones of Ammonites oxynotus and A. raricostatus, are elsewhere associated with indubitable Lower Liassic species in strata which, though superior to the " Bucklandi-beds," do not admit of a division into zones of life ; and it is not certain whether these strata, which may be designated as the Belemnite beds of the Lower Lias (or the zone of Belemnites acutus), represent the whole of the beds superior to the zone of A. Bucklandi, or whether only a portion of the upper series is present, Beds of this character occur in Shropshire, near Bath, at Ballintoy, Co. Antrim, in the east of France, &c.

It is not my present intention to discuss the palaeontological affinities of the whole Lower Lias and Middle Lias, which I reserve till such time as the claims of the so-called recurrent species have been fully investigated ; but so far as the distribution of species in the uppermost beds of the Lower Lias, and in the lowest horizon of the Middle Lias is concerned, there appears to be a palaeontological unconformity between the Lower and Middle Lias as herein defined. If this be true, then the lithological conditions will justify me in suggesting a break in the stratigraphical succession. It is a received axiom that a change in lithological conditions is accompanied by