part 1] OE THE SHALES-WITH-'BEEF.' 85 



need only be pointed out that Prof. Haug l already recorded this 

 appearance of cryptogenous types at the base of his Lotharingian 

 (namely, his zone of Microderoceras birchi, in which he includes 

 Pararnioceras nodosa ries of the alcinoe bed). -- 



Now, the appearance of such new types is often due to changes 

 in the shore-line or transgressions of the sea. Such, however, 

 cannot be proved in the present case, although there may be dis- 

 continuities in the succession in Dorset, as, for instance, at the 

 horizon of the alcinoe bed, which, with its large ammonites, 

 presents the appearance of a condensed stratum, suggestive of 

 redeposition in situ, and is the only bed in which Dr. Lang found 

 a Qryplioea ('of the G. incurva fades'). Other beds of friable 

 marl occurring sporadically in the succession may also indicate dis- 

 turbance by current-action or periods of temporary emergence. The 

 neighbouring Mendip axis, of course, was continually oscillating, 

 and the mixture of Agassiceras and Arnioceras with Arietites 

 and Microderoceras (known from parts of Gloucestershire where 

 Pararnioceras seems to be absent) suggests a composite bed with 

 several non-sequences above the sauzeanum subzone, due to this 

 continual movement. That is to say, in Gloucestershire there 

 probably occur only a few remnants of the various horizons here 

 described. In any case, the series of beds under consideration 

 represent a shallow-water formation. During the intervals of 

 submergence, there was in Dorset rapid deposition, indicated by 

 the crushing of the shells ; and some layers particularly rich in 

 calcium carbonate may have formed concretions or continuous 

 limestones, after the manner of those that Prof. Walther 3 records 

 as forming between the tide-marks at Suez. 



The fact that ammonites are common in these shallow- water 

 formations need not cause surprise. Many ammonites probably 

 lived very much like the recent Nautilus, chiefly crawling at the 

 bottom, but able to swim well and quickly, 3 and doubtless most of 

 the ammonite families had their home in the comparatively shallow- 

 water regions of the continental shelves. Lytoceratidse and Phyllo- 

 ceratidse were probably pelagic, and preferred warm surface-water 

 or currents 4 ; which supposition may explain why they are not 

 confined to the Mediterranean regions, but occasionally became 

 abundant elsewhere, as in the Yorkshire Upper Lias or the Dorset 

 Inferior Oolite, in neither of which areas the water can have been 

 very deep. On the other hand, the bituminous character of some 

 of the shales and their occasional richness in metallic sulphides 

 might suggest that the conditions were like those prevailing in the 

 Black Sea at the present day : namely, a superficial layer of water 

 of low specific gravity, with abundant nectonic and planctonic 

 organisms, and ' regions of death ' below, rich in salts, poor in 



1 Traite de Goologie, vol. ii, fasc. 2 (1910) p. 954. 



2 ' Emleitung- in die Geologic, &c.' pt. iii (1894) p. 699. 



:i L. F. Spath, 'Notes on Ammonites ' Geol. Mag. 1919, p. 32, footnote (3). 



4 Id. ' Jurassic Ammonites from East Africa, &c.' Geol. Mag. 1920, p. 362. 



