Vol. 52.] A DELIMITATION OF THE CENOMANIAN. 151 



Pecten elongatus, Lam. 



There is much doubt about this species, because the figures given 

 by d'Orbigny and Goldfuss are not alike, and because d'Orbigny's 

 description does not agree with his figure. We take d'Orbigny's 

 description as the best guide (< Pal. Fr.' p. 607), and from this we 

 learn that it has 30 to 40 unequal ribs, sometimes alternating 

 large and small ribs, sometimes grouped in threes (one large and 

 two small ones), each having prominent lamellose scales at intervals. 

 His figure, however, does not express these characters at all clearly. 

 We have found specimens at La Heve which agree with his descrip- 

 tion exactly and are also identical with specimens from the Lower 

 Chalk of England which we have been accustomed to regard as 

 P. elongatus. In all of them there is a marked tendency for the 

 ribs to be arranged in threes, a large one in the centre with a small 

 one on each side. 



Dr. Barrois states 1 that this species is identical with the form 

 figured under the name of P. cretostis by Goldfuss (' Petr. Germ.' 

 pi. xciv. fig. 2), and the figure certainly agrees with d'Orbigny's 

 .description. P. elongatus of Goldfuss is a Tertiary species. 



It differs from P. Marrotianus in not having two minor ribs 

 «between each of the groups of three, in being much less regular, and 

 in having much more prominent scales. We have not seen this 

 species from the Cenomanian of Devon, though it ought to occur 

 there. 



Pecten cf. Puzosianus, Math, in d'Orb., ' Pal. Fr. Terr. Cret.' vol. iii. 

 p. 610, pi. 437. figs. 1-4. 



This is another case in which d'Orbigny's figure and description 

 ■do not entirely agree. The figure shows perfectly plain ribs of 

 unequal breadth, but set close together, interrupted only by con- 

 centric lines of growth ; yet d'Orbigny says that the ribs are fur- 

 nished on the sides with imbricated plates : he adds, however, that 

 these plates are wanting in the middle of the shell, ' but may there 

 have been abraded.' 



We have specimens both from France and from Devon which 

 much resemble the figure, except that their ribs are rather fewer, 

 flatter, and more strongly marked, but there is no trace of their 

 ever having borne imbricating plates ; possibly these were very 

 delicate and easily removed. From its occurrence in the typical 

 Cenomanian country we think that this must be the shell described 

 by d'Orbigny. 



Pecten subacuttjs, Lam. in d'Orb., ' Pal. Fr. Terr. Cret.' vol. iii. 

 p. 605, pi. 435. figs. 5-10. 



We had found a Pecten in the lowest part of the Devon Cenoma- 

 nian which seemed to be this species, but its state of preservation 



1 ' Mem. sur le Terr. Cret. des Ardennes,' Ann. Soc. geol. du Nord, vol. v. 

 .(1878) p. 318. 



