ISOARCA. 65 
Genus—Isoarca, G. Minster, 1842. 
[‘ Neues Jahrb. fiir Min., &.,’ 1842, p. 98; ‘ Beitr. zur Petrefact.,’ pt. vi, 1843, p. 81.] 
Tsoarca AGassizi1, Pictet and Rouv, 1852. Plate XIII, figs. 7a, b. 
1842, Crromya crassicornis, L. Agassiz. Et. Crit. Moll. Foss., p. 36 (partim), 
pl. vii f, figs. l—4 (non 5—10). 
1850. Isocarpra crassrcornis, A. d’Orbigny. Prodr. de Pal., vol. ii, p. 187 
(partin). 
1852. Isoarca AGassizu, F&F. J. Pictet and W. Roux. Moll. Foss. Grés Verts de 
Geneve, p. 466, pl. xxxviii, fig. 3. 
1866. — _— F. J. Pictet and G. Campiche. Foss. Terr. Crét. Ste. 
Croix (Matér. Pal. Suisse, ser. 3), p. 422. 
1875. —_— _ A. J. Jukes-Browne. Quart. Journ. Geol. Soe., vol. 
xxxi, p. 200, pl. xv, figs. L—3. 
1879. — canrapricienstis [H. G. Seeley, MS.], # G. H. Price. The 
Gault, p. 60 (nom. nud.). 
The only examples which I have seen of this species are a few internal moulds 
found in the Cambridge Greensand (derived), 
Isoarca opesa (d’Orbigny), 1844. Plate XIII, figs. 8 a—c. 
1844, NucuLa opesa, A. d’Orbigny. Pal. Frang. Terr. Crét., vol. iii, p. 180, pl. 
ecciv, figs. 10—14. 
1846. Isocarpra Orpianyana, 4. d’Archiac.. Mem. Soc. Géol. France, ser. 2, vol. 
li, p. 305, pl. xv, fig. 1. 
1847. Nvucuna rnoromacGensis, d’Orbigny. Pal. Frang. Terr. Crét., vol. ili, p. 766. 
1848. Arca 1socarprzForMis, H. P. Nyst. Tableau Espéces vivant et foss. 
des Arcaces, p. 38. 
1850. Isoarca opesa, A. d’Orbigny. Prodr. de Pal., vol. ii, p. 168. 
1866 _ — F.J. Pictet and G. Campiche. Voss. Terr. Crét. Ste. Croix 
(Matér. Pal. Suisse, ser. 3), p. 423. 
Non 1837. Nucwuna ovesa, A. Gold/uss. Petref. Germ., vol. il, p. 150, pl. exxiv, fig. 4. 
Description.—Shell oval or slightly subquadrate, rounded, length greater than 
height, much inflated, short anteriorly, long posteriorly; ventral margin 
slightly curved. Umbones large, anterior, close together, curved forward. 
Hinge-line about two-thirds the length of the shell. Hinge-area indistinctly 
limited, short anteriorly, elongate posteriorly. Ornamentation consists of 
numerous, very regular, closely-set, concentric grooves, with broader, flattish 
interspaces ; and radial grooves which are less distinct than the concentric; the 
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