452 ^^^^^jffi^^^WS^^^^^^^^ [vol. Ixxviii, 



(D ) Upper and part of the Middle Lias Succession^, and 

 Correlation. (^S. S. 13.) 



In his examination of Prof. Watson's collection Dr. Spath was 

 •able to make certain notable additions to the recognizable horizons 

 of the Upper Lias : he was able to place as chronological indices 

 two new genera which I had recently named and iilusti-ated in 

 ' Type- Ammonites " — SiJdaites (pi. ccxvii) and Ilildoceratoides 

 {pi. ccxviii). Fm'ther, he distinguished a horizon by the name 

 <inguinum, and -p\a.ce([ Sar2)oce/'afoides as marking a distinct date: 

 •species of this genns had been obtained by ^ih\ J. F. Jackson from 

 the Junction-Bed of the Dorset Coast (see p. 44:3) ; but on that 

 ■evidence alone it had not been considered advisable to distinguish 

 Sarpocerotoides as a separate tune-term. Xow all this evidence 

 falls nicely into line. 



Adopting Dr. Spath' s terms, and using information otherwise 

 ■obtained, it is possible to present the folloT\ing sketch of the chron- 

 ological sequence in the L^pper Lias (and top of the Middle Lias) 

 — Table YI. p. 453. At the same time, a series of stratigraphical 

 terms are appended which may be useful to the memory, as well as 

 indicating where certain beds are well exposed. It is not intended 

 to say that the beds are only exposed at the places which give their 

 names : it is known, for instance, that T\'hitby possesses far more 

 than the strata of the three hemerse which stand opposite ' Whitby 

 Beds.' But, on the other hand, deposits of certain hemerse seem 

 only to have been preserved at the localities which give their 

 names : thus the sti'ata of pseudovaium date are only known in 

 Yorkshh-e. 



A similar phenomenon attends exaraiuiii : this species has not 

 been recognized outside of Y'orkshii-e. Several of the associates of 

 exaratum in the Jet-Eock of Y'orkshu'e have now been found in 

 other localities — ^for instance, in the LeptcEua Bed of Somerset, 

 where the inch-to-inch collecting gives them a certain sequence, 

 which, however, must be taken with some reserve : fii'st, because the 

 present examination is necessarily rather cursoiy, and, secondly, 

 because the specunens are somewhat crushed and ill-preserved. But 

 the important fact is that, while the Lepfcena Bed yields these 

 exaratum associates, another of the Jet-Eock genera (B-o/ipo- 

 ceratoides) is found in a bed of different lithic character at a 

 distinctly higher level — separated from the Leptcena Bed by the 

 Fish-Bed. Therefore, it may be argued that the Yorkshire Jet- 

 Rock is not a deposit of one date, but is polyhemeral. 



The exact chronological position of exaratum is, then, uncertain. 

 There is Httle doubt that it has some affinity with the forms now 

 spoken of as ' G-rantham ammonites," and it is therefore concluded 

 that its date is either just before or just after those forms. If 

 placed before the Grantham ammonites, a non-sequence in the 

 Lept(Ena Bed is produced, and a considerable non-sequence in the 



