PECTEN. 149 



specimens whicli I have seen, the concentric ornament on the right valve is indis- 

 tinct, owing to the imperfect preservation of the surface of the shell, but in several 

 cases the fine concentric grooves are clearly shown on the left valve. 



Fecfcn Darius, d'Orbigny,^ from the Albian, is a closely allied foi-iii, l)ut at 

 present is known only by the brief description in the ' Prodrome.' The form from 

 the Gault of Cosne, described by De Loriol ^ as P. Darius, appears to differ from 

 P. orhicularls in the inequality of its ears ; the figures do not show the character 

 of the ornamentation satisfactorily. Amusiuni sulcatellum, Stoliczka,^ and P. 

 concciitiic('-suIcati(.'<, Miiller,* appear to be closely allied to P. orhicularis. 



The Senonian specimens described by Goldfuss, Holzapfel, etc., as I\ laminosns, 

 Goldfuss, and by Favre and Bolim as P. suhlaminosus, seem to agree with those 

 forms of P. orbicularis which have more numerous ribs tlian usual, and pai'ticularly 

 with some examples from the Warminster Greensand and the Chloritic Marl of Maiden 

 Bradley. Professor Holzapfel has kindly sent me eight specimens from the Aachen 

 Greensand, but it is difficult to compare them satisfactorily with English examples 

 owing to their different mode of preservation. There does not, however, appear to 

 be sufficient reason to regard them as distinct from the Cenomanian forms of 

 P. orhicularis. The ears in the example figured by Goldfuss are larger than is usual 

 in P. orbicularis. 



P. membranaceus, Nilsson,'' is similar in form to P. orbicularis, but has the con- 

 centric ornamentation very fine, so that the shell appears to be almost smooth. 

 P. Nilssoni, Goldfuss,'^ is also distinguished by being nearly smooth, and (judging 

 from Goldfuss' fig. 8 b) is still further separated from this group by its deep byssal 

 sinus. 



P. numriinlaris, Fischer de Waldheim,^ is a closely allied form, but without 

 seeing a series of specimens I am unable to make a comparison. 



P. {Anmsium) balticus. Dames,® is probably identical with ./'. orbicularis. 



1 'Prodr. de Pal.,' vol. ii (1850), p. 139. 



2 " Faune du Gault de Cosne," 'Mem. Soc. Pal. Suisse,' vol. ix, 1882, p. 84, pi. x, fig. 6. 



3 'Palseout. ludica, Cret. Fauna S. India,' vol. iii (1871), p. -136, pi. xxxi, figs. 12, 17. 

 ^ 'Mollusk. Uutersenon von Braunschweig u. Ilsede' (1898), p. 34, pi. v, fig. 9. 



5 ' Petrif. Succ' (1827), p. 23, pi. ix, fig. 16 (lower figure). See also Hennig, Holzapfel, Zitk-l, 

 Greiuitz, Goldfuss, etc. 



« 'Petref. Germ.,' vol. ii (1836), p. 76, pi. xcix, fig. 8. 



' Fischer de Waldheim, 'Bull. Soc. Imp. Nat. de Moscou,' vol. xvi (1843), p. 135, ].l. v, fig. 4; 

 d'Orbigny, in Murchison, de Verneuil, and Keyserling, ' Gcol. de la Eussie,' vol. ii (1845), p. 475, 

 pi. xli, figs. 20—23, and figs. 16—19 (P. demissus) ; d'Orbigny, ' Prodr. de Pal.,' vol. i (1849), p. 373; 

 Trautschold, ' Bull. Soc. Imp. Nat. de Moscou,' vol. xxxviii (1865), p. 23, pi. iii, fig. 2 ; Nikitin, " Les 

 Yestiges de la Pcriode Cret. dans la Euss. Centrale," ' Mem. Com. Geol.,' vol. v, 1888, p. 73 ; 

 P. demissus, Trautschold, ' Bull. Soc. Imp. Nat. de Moscou,' vol. xxxiv (1861), p. 268, pi. vii, fig. 4. 



8 'Zeitschr. d. deutsch. geol. Gesellsch.,' vol. xxvi (1874), p. 762, pi. xxi, fig. 1. Nulling, 

 " Baltischen Cenoman.," ' Palaeont. Abhandl.,' vol. ii, p. 17, pi. ii, fig. 7. 



