Vol. 52.] IN YORKSHIRE AND LINCOLNSHIRE. 183 



■of drift, 1 further penetrated pale-blue sandy clay or shale for 

 75| feet before reaching the water-bearing Coralline Oolite. This 

 shale evidently represents the lowest Kimeridgian beds ; the only 

 fossil that I was able to obtain from it was an indeterminable 

 fragment of a small slender belemnite, distinct from any of the 

 Speeton species known to me. 



In my former paper I described the strikingly sudden change of 

 fauna which takes place at the top of the zone of Belemnites lateralis. 

 Immediately below this horizon the prevalent ammonites are the 

 deep-whorled, round-backed forms of the genus Olcostephanus and its 

 allies, and the belemnites all belong to the short thick lateralis- 

 group, while immediately above it the clays abound in the flat- 

 whorled, square-backed ammonites of the genus Hoplites and the 

 slender hastate belemnites of the jaculum-type. So complete is this 

 change that I was in doubt when previously describing it whether 

 the apparent mingling of the forms at the junction of the zones 

 in the ' Compound-Nodule Band ' (D 1) really indicated contem- 

 poraneity, or whether through lack of sedimentation the dead 

 remains of the older fauna had lain uncovered on the sea-floor long 

 enough to become embedded in the same layer with the first relics 

 of the new species. 2 I have, however, since found two specimens 

 of the lateralis-type of Belemnites (B. subquadratus, Roem.) dis- 

 tinctly within the zone of B. jaculum, the first in the clay imme- 

 diately overlying D 1, and the second in the clay 18 inches above 

 that band, which proves that the older type was not quite extinct 

 on the appearance of the newer, though very nearly so. 



The researches of Prof. Pavlow on the Speeton fossils have thrown 

 new light on the change at this horizon. He has been able to 

 demonstrate that the incoming fauna was one which had been de- 

 veloped and had prevailed in southern seas, while the displaced fauna 

 was markedly northern in its origin and range. 3 The southern 

 fauna remains almost pure throughout the ' noricus-be&s ' (C 11 to 

 ■C 7), but above that zone the Olcostephani reappear in a group of 

 species (centring around Ammonites speetonensis) which, while dis- 

 tinctly recalling the Olcostephani of the upper part of the lateralis-zone 

 (D 1 to D 3), are yet so completely modified as to be in every case 

 specifically different. These replace the Hojolites-tj^e of ammonites, 

 and though the hastate belemnites (Belemnites jaculum and allies) 

 persist much longer, they also eventually disappear, and the field 

 is reoccupied by types (B. brunsvicensis) which have probably been 

 derived from ancestors pertaining to the lateralis-gTowp. 



The line of research thus indicated is still being pursued, and 

 promises to be rich in results bearing on the extent and character 



1 See Mem. Greol. Surv. ' Jurassic Kocks of Great Britain : Pt. I. Yorkshire,' 

 by C. F. Strangways, p. 375. 



2 'Subdivisions of the Speeton Clay,' Quart. Journ. Geol. Soc. vol. xlv. 

 <1889) p. 589. 



3 ' Argiles de Speeton,' pp. 93, 188 et seqq. (sep. cop.) and ' On the Marine 

 Beds closing the Jurassic and opening the Cretaceous, with the History of their 

 Fauna,' by A. Pavlow, in Bull. Greol. Soc. America, vol. iii. (1892) p. 61. 



