﻿Vol. 66.~] JURASSIC STRATA OP SOUTH DORSET. 83 



specimens that properly belong to the bifrons bed are often worn, 

 iron-coated, and deposited on edge. 1 



The striatidus layer appears to be quite a regular deposit ; but 

 then there is a big gap between the striatulus layer and the bifrons 

 bed — a time during which about 250 feet of strata were laid down 

 in the Cotteswolds. 



With regard to Day's Pleurotomaria Bed — the top layer of the 

 Marlstone 2 — I do not feel certain of having met with it, unless it be 

 the serrata bed mentioned above (p. 65). But, considering how 

 often certain beds are locally missing from the Junction Bed, it is 

 quite possible that it may be a layer just above the serrata bed, only 

 developed occasionally. In the Jermyn Street Museum there are the 

 following Ammonites from Upper Lias of Chideock [ = Junction Bed 

 of Down Cliffs] — B. No. 22475, Dactylioceras cf. tenuicostatum 

 (Young & Bird), and 22514, Dactylioceras crassiusculosum (Simpson). 

 The first of these is a species from the annulatus zone of Yorkshire, 

 and Martin Simpson records the second from the Jet Eock, which is 

 higher. It may be interesting, therefore, to compare the Dorset 

 and Yorkshire Toarcian. 



V. FlRV-STIiJATZ-LUS ToARCTAN. 



(a) Dorset and Yorkshire Coasts compared. 



The difference in development is remarkable. On the Dorset 

 coast the pre- striatulus Toarcian beds are packed into a seam of 

 calcareous stone about 2 feet thick. On the Yorkshire coast, ac- 

 cording to a useful section given by Martin Simpson, 3 they occupy 

 nearly 200 feet. An epitome of Simpson's section and divisions of 

 the Yorkshire Toarcian is given on p. S-4 (Table IV), with the 

 dates, according to my interpretation, at the side. Tate & Blake's 

 rendering is also given, correlated with Simpson ; the correlation 

 is fairly obvious, except with regard to Beds 1-10, where they 

 have a far greater thickness. Alongside is the development of 

 the Dorset strata of the same dates. 



Simpson's Division 1, by the species recorded and known to come 

 from there, was deposited during five hemeroe — dispansi-bifrontis. 

 The bulk of the strata of the division probably belong to the date 

 of bifrons ; but there is good evidence for the others. For dispansi 

 hemera, Pldyseogrammoceras orbiynyi, S. Buckman (Monogr. p. 188) 

 and Ammonites gubemator, Simpson — a Lytoceratoid, probably 

 Alocolytoceras near to perlazve, Denckmann. For striatidus date the 

 evidence is abundant — the term ' striatulus shales ' is in use ; and 

 species of this facies are plentiful at the Peak. For variabilis-lilli, 

 the latter perhaps dubious, there is evidence, in the Whitby Museum 

 collection of types, of a rich Ammonite fauna: — A. obliquatus, 



1 The Red Bed and the Yellow Conglomerate Bed of Burton Bradstock 

 would conform much more to Blake's requirements for an aggregate deposit ; 

 they are made up of sweepings from deposits of various dates. 



2 Quart. Journ. Geol. Soc. vol. xix (1863) p. 288. 



3 ' Fossils of the Yorkshire Lias' 2nd ed. (1884) pp. ix-xiv. 



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