part 1] JURASSIC CHRONOLOGY : LIAS. 79 



Table VII. — Bevised Chronology relating to Ammonites tctbellus. 



Raasayan. 

 aplanatum 



lacdonnellii 5. 



Yorkshire. 



18. JEchioceras 

 aplanatum. 



20. JSchioceras 



macdonnellii. 



Gloucester- 

 shire. 



raricostatoides 



f4,e. Crassicostate 

 JEchiocerata. 



4cZ. JEchioceras, 

 cf. boreale. 



4c. Ammonites 

 tubellus. 



46. JSchioceras 



rhodanicum. 



20. Ammonites 

 tubellus. 



Ammonites 

 tubellus. 



I 4sa. Crassicostate 

 (,_ JEchiocerata. 



bispinigerum 3. 



20. Deroeeras aff. 

 miles. 



Dorset. 



103. Crassicostate 

 JEchiocerata. 



100. JSchioceras 

 cf. boreale. 



100. Ammonites 

 tubellus. 



100. JEchioceras 

 rhodanicum. 



99. Crassicostate 

 JEchiocerata. 



96. Deroeeras 

 miles. 



due to stratal failure ; and, whether the stratal failure is due 

 to penecontemporaneous erosion or to depositional failure, the 

 A.-tubellus episode remains just as distinct. 



That Ammonites tubellus has not been found in Scotland, 

 where the Echiocerate sequences are so conspicuous with their 

 abundant faunas, might be accounted for on the principle of 

 dispersal failure — that A. tubellus had a restricted range. And 

 the same might be said of the lack of this species from Continental 

 records. But this plea is somewhat discounted by the absence 

 of the species from Radstock (Somerset), intermediately situated 

 between Gloucestershire and Dorset — a locality where the Echio- 

 cerate faunas are very conspicuously developed. 



If, however, A. tubellus marks a distinct episode — possibly 

 a time of deep sea, with which the larval character of the species 

 would accord, then the absences from other areas might come 

 into line with that from Radstock and he attributable to stratal 

 failure, presumably caused by penecontemporaneous erosion. In 

 any case, it will be necessary to analyse the Echiocerate faunas 

 more precisely, in order to ascertain which species are pre- and 

 which ■post-tubellum — a long task that cannot be undertaken now, 

 but one promising interesting revelations. It is also desirable to 

 confirm the position of A. tubellus in Yorkshire. 



The descending sequence subplanicosta, densinodus, armatoids, 

 Echiocerates, is that which I suggested in my former paper, 

 p. 268. In the three southern cuttings this sequence shows no 

 lacunae, which is reasonable evidence for its correctness, 1 and, as 

 regards the sequence subplanicosta, densinodum, JSchioceras, this 

 is established ; for Mr. Richardson records it at Bayliss Hill. 



Stratal failure ma} r account for the absence of the armatoid 



1 The lacuna of Bishop's Cleeve has already been commented upon (p. 70). 



