392 CRETACEOUS LAMELLIBRANCHIA. 



the larger specimens a posterior "wing'-like part is cleveloped, and may be separated 

 from the remainder of the valve by a furrow. The surface is marked with 

 growth-rings. 



Right valve thin, slightly convex or nearly flat ; umbo small, with the marginal 

 part sometimes growing upAvards at a considerable angle with the earlier ]i:irt. 

 Surface nearly smooth. 



Affinities. — The English specimens agree closely with the figures given by 

 Miiller, and fairly well with some of the smaller examples figured by Coquaud ; 

 but the larger forms, in which the height of the shell is relatively less, have not 

 been found in this country. 



Specimens in which the attached surface is larger than usual and the height of 

 the shell relatively less (Plate LX, figs. 14, 15) resemble some forms of 0. rrslcu- 

 laris, and there seems to be almost a complete passage lietween these tAvo 

 species; this resemblance has already been noticed by Peron in specimens found 

 in the Lower Senonian of Tunis. 



The right valve of small specimens is similar to that of the forms of 0. incarva 

 in which the posterior curvature of the umbo is slight. 



The form from Tebessa (Constantine), named 0. tevesthensis by Coquand, Avas 

 subsequently identified by that author Avith 0. Boiicherom, and that identification 

 has been supported by Peron. 0. liouvilici, Coquand,* from Algeria, resenil)les 

 the forms of 0. Bonclicroui which have a short and high shell. 



Remarls. — In England the specimens noAV referred to 0. Boiu-ln'niui liaA'e been 

 hitherto identified as 0. Wegmanniana, d'Orbigny,^ l)ut the latter species is 

 regarded by Peron^ and l)y Leriche,' as a synonym of 0. iucnrra (p. 088). So 

 far as I liaA^e been able to make out the characters of 0. Wcgmanniaiui, from the 

 figures of d'Orbigny and Coquand and from the examination of a single right 

 A'alve, I am inclined to accept the view of Peron and Leriche. 



This species is often gregarious ; most of the English examples are of rather 

 small size, and the left valve is much more frequently fomid than the right vahe. 



Type. — From the LoAver Senonian of Lavalette (Cliarente), said to be in the 

 l^cole des Mines, Paris. 



Diatiiliiition. — Zone of Ilohixti'V plama^ of Hitchin. Recorded hy Rowe from 

 the Isle of Wight. 



Zone oi Micrader cor-duijitiiiiiin of Whitway (Hants), Farningham Road, Preston 

 near Faversham, Gravesend, Charlton, and Loam Pit Hill (Lewisham). 



> 'Geol. Pal. reg. sml. Province Coustantine ' (1862), p. 232, pi. xxii, figs. 9—10; aud 'Mem. 

 Ostrea, Terr. Crct.' (1809), p. 89, pi. xxi, figs. 3—6, pi. xxiv, figs. 7—11. 



2 'Pal. Franc. Terr. Crt't.,' vol. iii (1847), p. 749, pi. cccclxxxviii, figs. 6—8. Coquand, op. cit. 

 (1869), p. 53, pi. iv, figs. 9 -11,'pl. xxiii, figs. 11 — 14. Alessandri, ' Pal. Italica,' vol. iv (1899), p. 198. 



•' ' Hist. Terr. Craie, S.E. Bassiu Anglo-Parisien ' (1888), p. 173. 



■' 'Compte Kendu Assoc. Fran?. Av. Sci.,' 1907, pt. 2 (1908), p. 338. 



