ISOARCA. 65 



Genus — Isoarca, G. Miinster, 1842. 

 [•Neues Jahrb. fur Min., &c.,' 1842, p. 98; ' Beitr. zur Petrefact.,' pt. vi, 1843, p. 81.] 

 Isoarca Aoassizii, Pictet and Boux, 1852. Plate XIII, figs. 7 a, b. 



1842. Ceromya crassicornis, L. Ayassiz. Et. Crit. Moll. Foss., p. 36 (parrim), 



pi. v'riif, figs. 1 — 4 {non 5—10). 

 1S50. Isocardia crassicornis, A, d'Orbiyny. Prodr. de Pal., vol. ii, p. 137 



(partim) . 

 1852. Isoarca Aoassizii, F. J. Pictet and W. Boux. Moll. Poss. Gres Verts de 



Geneve, p. 4Gb', pi. xxxviii, fig. 3. 

 ]866. — — F. J. Pictet and G. Campiche. Poss. Terr. Cret. Ste. 



Croix (Mater. Pal. Suisse, ser. 3), p. 422. 

 1S75. — ■ — A. J. Jukes- Browne. Quart. Journ. Geo]. Soc, vol. 



xxxi, p. 300, pi. xv, figs. 1—3. 

 1879. — cantabrigiensis [II, G. Seeley, MS.], F. 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 obesa (d'Orbigny), 1844. Plate XIII, figs. 8 a — c. 



1844. JS'ucula obesa, A. d'Orbiyny. Pal. Franc. Terr. Cre't., vol. iii, p. 180, pi. 



ccciv, figs. 10 — 14. 

 184G. Isocardia Orbignyana, A. d'Archiac. Mem. Soc. Geol. Prance, ser. 2, vol. 



ii, p. 305, pi. xv, fig. 1. 



1847. Nucula rhotomagexsis, d'Orbiyny. Pal. Franc. Terr. Cret., vol. iii, p. 76G. 



1848. Arla isocardijjfohmis, II. P. Xyst. Tableau Especes vivant et ibss. 



des Arcaces, p. 38. 

 1850. Isoarca obesa, A. d'Orbiyny. Prodr. de Pal., vol. ii, p. 163. 

 1866 — — F. J. Pictet and G. Campiche. Foss. Terr. Cret. Ste. Croix 



(Mater. Pal. Suisse, ser. 3), p. 423. 



Non 1837. Nucula obesa, A. Goldfuss. Petief. Germ., vol. ii. p. 150, pi. exxiv, tig. 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, flatfish 

 interspaces; and radial grooves which are less distinct than the concentric; the 



9 



