TJOPTRTHA. 257 



Volgian of Moscow, wliich were identified by Prof. Pavlow, l)ut their resemblance 

 to Rouillier's fio-m-e is not quite so close. Some of the smaller specimens approach 

 P. peregriaa (d'Orbigny'), which is said to differ from /'. Orbujmana by smaller 

 curvature of the ventral margin, the presence of a shallow depression extending to 

 the ventral margin below the umbones, and l)y the ornamentation consisting of 

 radial rows of minute tubercles instead of linear ribs. One specimen from Spilsby, 

 however, shows both types of ornamentation, suggesting that the two species are 

 not really distinct. The hinge is not seen in any of the English specimens of /'. 

 Orhirjniaiia, but the figure given liy Rouillier, although not fpiite satisfactory, is 

 suggestive of I'miopea; the form of the shell, however, resembles that of some 

 Jurassic species of Flenromiin. 



DistrilmtloiL. — Spilsby Sandstone (zone of Belemuitus laturalls) of Dounington. 



Famil I/— FOnOMYAGIDM, Dall. 

 Qeniis — LiorisTHA, F. B. Meel; 1864. 



(' Check List Invert. Foss. N. America,' pp. 12, 32 ; T. A. Conrad in Kerr's ' Keport Geol. Survey 

 N. Carolina,' vol. i, 187.5, Appendix A, p. 28 ; Meek, ' Invert. Cret. and Tert. Foss. U. Missouri,' 

 1876, p. 227.) 



Section — PsiLOMYA, Meek, 187G. 

 (Ibid., p. 229.) 



LiopiSTU.v (PsiLo.MY.\) g[G.\ntd:.v [Son-erhij), 1818. Plate XLIII, figs. ;5, 4; Plate 



XLIV, figs. 1, 2. 



1811. CoRBULA ?, /. Parkinson. Or;:,'anic Eemaius, vol. iii, p. 226. 



1818. — GiGANTEA, J. Sowerby. Miu. Couch., vol. iii, p. lo, pi. ccix, 



figs. 5—7. 

 1850. Pholadomya — A. d'Orhiijnij. Prodr. de Pal., vol. ii, p. 157. 

 1854. Thetis — J. Morris. Cat. Brit. Foss., ed. 2, p. 227. 



18(3.5. _ ? _ F. J. Pidet and G. Campiclie. Ten-. Crct. Ste. Croix 



(Matc'r. Pal. Suisse, ser. 4), pp. 199, 210. 

 1870. POROMTA ^ — F. StoUczka. Palseout. Indica, Cret. Fauna S. India, 



vol. iii, p. 42. 

 1882. LiOPiSTHA — P. de Loriol. Gault de Cosne, p. 45, pi. vi, figs. 1—5. 



1 Murchison, Verneuil, and Keyserling, 'Geol. de la Russie de I'Europe,' vol. ii (1845), p. 468, 

 pi. xl, figs. 10—12. The hinge is figured by Zittel, ' Handbuch d. Paliiont.,' vol. ii, p. 125. fig. 179, 

 and ' Gruudziige d. Paliiont.,' ed. 2 (1903), p. 330, fig. 732. 



34 



