﻿Vol. 66.~\ JURASSIC STRATA OP SOUTH DORSET. 87 



Prom the foregoing Table it will be seen that Mr. Thompson finds 

 the greatest development (in the way of faunal change) of Upper 

 Lias, in Northamptonshire, to be in the strata once called bifrons 

 or communis beds. Here he makes five divisions, and names three 

 zones — braunianum, fibulatum, subcarinatum. There is good reason 

 to suppose that all these zones could be detected in Yorkshire ; only 

 that they are more obscure from paucity of sediment. But in York- 

 shire Simpson's Divisions 2, 3, & 4 (some 50 feet of strata) appear 

 to be a development not found in Northamptonshire ; for this series 

 I was proposing a zonal name, and Ammonites ' ovatus' will have 

 to do duty temporarily, though the identification is incorrect (see 

 above, p. 85). 



Below the ' ovatus ' bed of Yorkshire is the mulgravius bed, and 

 below this again the Jet Rock series with A. exaratus, etc. It is 

 evident from the Yorkshire strata that there are two zones, and I 

 was preparing to name them in this paper. I find that Mr. Thomp- 

 son, from his study of the North am pton shire strata, has reached 

 the same conclusion independently, and has named the lower zone 

 latescens zone. I hesitate to adopt this : a study of Yorkshire 

 types throws much doubt on identifications of Ammonites latescens, 

 and on its horizon ; while of A. exaratus the type is definitely 

 known, and so too its bed. 



Below the exaratum zone (Jet Bock) of Yorkshire is the deposit 

 known as the ' Grey Shales ', or annulatus zone of Tate & Blake. 

 As the species is not A. annulatus, but is A. ienuicostatus, Young & 

 Bird, a change of name is desirable. Below the latescens zone of 

 Northamptonshire is a small deposit of paper-shale : and below the 

 equivalent of the exaratum zone of Gloucestershire is the deposit 

 called the Leptama Beds, which are certainly above the acutum 

 layer. The suggestion may, then, be made that the Leptcena Beds 

 of Gloucestershire and the South- West of England are of about the 

 same date as the tenuicostatum zone of Yorkshire, and that this 

 zone is later in date than the acutum zone or Transition Bed of 

 the Midlands. It may be admitted that this is at present only a 

 suggestion based on stratigraphical evidence, and that the faunal 

 evidence is mainly negative — that is to say, that the fossils of the 

 acutum zone and of the tenuicostatum zone are different, implying 

 that the zones are sequential, not contemporaneous. So here is a 

 working hypothesis : to prove or disprove it further evidence is 

 required. 



(c) Migration of Areas of Maximum Deposit. 



The migration from north to south of the area of maximum 

 development of the Toarcian strata in England is an interesting 

 phenomenon. It seems to be a regular progress from earliest beds 

 in the north to latest beds in the south ; but, no doubt, further 

 knowledge will show some irregularity. Present results are given 

 in the accompanying Table (YI, p. 88) : — 



