FROM NORTHWESTERN EUROPE 33 
Description. External characters. L. vaga is weakly trilobate with a low, 
symmetrical uniplication in the anterior commissure. The brachial is the more 
inflated valve and the species is generally characterised by a rather globose appear- 
ance. Concentric ornament is only rarely present and consists of weak growth 
lamellae and numerous fine growth lines. The beak is typically small and sub- 
erect ; conjunct deltidial plates delimit the small submesothyridid pedicle opening. 
Beak ridges and an interarea are present but only poorly developed. 
Fic. 13. Transverse serial sections of Lacunosella arolica (Oppel). Oxfordian. Chavoley 
Beds. Virieu-le-Grand (Ain), France. (x3). 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
2°69 cm 2:00 cm 3°02 cm 
2°84 cm 1-84 cm 3:06 cm 
2:78 cm I-g0 cm 2°97 cm 
