392 CRETACEOUS LAMELLIBRANCHIA. 



the larger specimens a posterior wing-like part is developed, 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 upwards at a considerable angle with the earlier part. 

 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 Coquand ; 

 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. reticu- 

 laris, and there seems to be almost a complete passage between these two 

 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. incurva 

 in which the posterior curvature of the umbo is slight. 



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

 subsequently identified by that author with 0. Bouchcvoni, and that identification 

 has been supported by Peron. 0. Bouvillei, Coquand, 1 from Algeria, resembles 

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



Remarks. — In England the specimens now referred to 0. Boucheroni have been 

 hitherto identified as 0. Wegmanniana, d'Orbigny, 2 but the latter species is 

 regarded by Peron 3 and by Leriche, 1 as a synonym of 0. incurva (p. 388). So 

 far as I have been able to make out the characters of 0. Wegmanniana from the 

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

 valve, 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 found than the right valve. 



Type. — From the Lower Senonian of Lavalette (Charente), said to be in the 

 Ecole des Mines, Paris. 



Distribution. — Zone of Holaster planus of Hitchin. Recorded by Rowe from 

 the Isle of Wight. 



Zone of Micraster cor-anguiaum of Whitway (Hants), Farningham Road, Preston 

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



1 'Greol. Pal. reg. sud. Province Constantine ' (1862), p. 232, pi. xxii, figs. 9 — 10; and * Mon. 

 Ostrea, Terr. Cret.' (1869), p. 89, pi. xxi, figs. 3—6, pi. xxiv, figs. 7—11. 



2 'Pal. Franc;. Terr. Cret.,' vol. iii (1847), p. 749, pi. ceeclxxxviii, figs. 6 — 8. Coquand, op. cit. 

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



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



* 'Compte Rendu Assoc. Franc;. Av. Sci.,' 1907, pt. 2 (1908), p. 338. 



