Vol. 52.] A DELIMITATION OF THE CENOMANIAN. 147 



and we have compared them with a number from different localities 

 in Prance. There is no doubt about the identity of the English 

 and French forms, and it is desirable that Sowerby's name should 

 be recognized in France as it is in England and Germany. Bh. con- 

 veoca, Sow., also occurs in the French Cenomanian. 



Rhynchonella Wiestii, Quenst. (1871), and Davidson (1874), in 

 Supplement to * Brit. Foss. Brach.' p. 66, pi. viii. fig. 31. 



The shells referred by Davidson to this species are very common 

 in Bed 13, the sandy chalk which overlies the zone of Ammonites 

 Maiitelli in Devon. It is a much more variable species than Davidson 

 seems to have supposed, for he describes it as having ' about 30 or 32 

 rounded ribs,' and adds that it approaches most nearly to Bh. Gra- 

 siana, *• of which it may perhaps be a large variety.' 



Having collected many specimens from this bed wherever it is 

 found, we cannot agree with Davidson. The shell seems to us 

 much more closely allied to Bh. Cuvieri, from which it can only be 

 distinguished by having, as a rule, fewer and larger ribs. The 

 average number seems to be 24 or 26, but there are forms which, 

 have as few as 18 and others which have as many as 30 ; the 

 former resemble Bh. Mantelliana, except that the ribs are not 

 angular, and the latter come so near to the broader varieties of 

 Bh. Cuvieri that, when placed beside them, they are indistin- 

 guishable. 



Although both the names Cuvieri and Mantelliana have been 

 admitted into our list, we believe the specimens so named are- 

 extreme varieties of one species, which may be called Bh. Wiestii, 

 and may be regarded as the ancestor of Bh. Cuvieri and Bh. reed- 

 ensis, Eth., which occurs still higher in the zone of Holaster planus. 



Terebratula tornacensis, d'Arch., Mem. Soc. geol. Fr. ser. 2 r 

 vol. ii. p. 316, pi. xviii. fig. 3, and varieties figs. 4-5. 



This species was founded on specimens from the Tourtia of 

 Tournay, and as this deposit is now known to be the littoral facies 

 of the Cenomanian in Hainaut, and of later date than the zone of 

 Ammonites inflatus, the occurrence of Tourtia forms in the Ceno- 

 manian of Devon and Sarthe is not surprising. 



When Davidson first described the Faringdon fossils in 1852 

 he identified certain forms as T. tornacensis, var. Boemeri, d'Arch., 

 though he was evidently for some time in great doubt about them 

 (see ' Brit. Cret. Brach.' vol. i. p. 62). At that time he imagined 

 that the Faringdon Sand was of Upper Greensand age, and, as the 

 Tourtia was then supposed to be a sort of combined Lower and 

 Upper Greensand, he saw no reason why a Tourtia species should 

 not occur at Faringdon. 



It seems, indeed, to be a fact that some of the forms of T. depressa 

 and T. tornacensis occurring in the Tourtia are practically indistin- 

 guishable from varieties of T. depressa and T. sella which occur in 

 the Lower Greensand of England ; but the typical form of T. iorna- 



Q. J. G. S. No. 206. m 



