THETIRONIA. 107 



yet they are occasionally al)sent. Otlici* differences arc seen in the very thin and 

 punctate shell, and in the internal ril). 



Nothing closely resembling the internal rib of Thetironia seems to be known in 

 any other laniellibranch. Bnt in some Jurassic and Cretaceous ^ species of Protn- 

 airdia a rib, either single or A-shaped, is found at the inner Ijoundary of the 

 posterior area ; this, however, differs from the rib of TJidironia, in that it reaches 

 the margin of the valve and is not continued anteriorly to the neighbourhood of 

 the anterior adductor. 



It seems, thei'efore, that although Thetirouia resembles the Cardiidic in several 

 respects, yet the points of difference are too great to allow of its being included in 

 that family. 



TiiETiiiDXiA JiiNuu (Suiceyhii), 1820. Plate XXV, figs. \oa—c; Plate XXVI, 



figs. 1—8. 



1822. Venus, G. Mantell. Foss. S. Downs, p. 73. 



1826. Thetis minor, /. de C. Soiverhtj. Min. ConcL., vol. vi, p. 21, pi. d.viii, 



fiys. 6 (? 5). 

 1829. — —M.J. L. Beftance. Diet. Sci. Nat., v,.l. liv, p. 275. 



1841. — SowEEBii, F. A. Rijmcr. Die Versteiii. d. iiord-deutscli. Kreidugeb. 



p. 72 (jiartim). 

 1845. — — var. a minor, var. /3. major. E. Forbes. Quart. Joiiru. 



Geol. Soc, vol. i, p. 242. 

 184G. — L^viGATA, A. d'Orhi(jinj. Pal. Frauy. Terr. Crct., vol. iii, p. 452, 



pi. ccclxxxvii, figs. 1 — 3. 

 1832-53. — MiNOE, G. P. Dcshaijcs. Traitii Elciueut. Conchyl., vol. i, p. 575, 



pi. xxii, figs. 3, 4. 

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



— — — S. P. Woodward. Manual of the Mollusca, fig. 221 (on 



p. 318). 



1865. — —F.J. Pictet and G. Cavijjkhe. Foss. Terr. Crct. Ste. Croix 



(Mater. Pal. Suisse, scr. 4), 

 p. 202, pi. cxii, fig. 4. 



1870. — LAEVIGATA, Pick'l uiul Cuiiqiiche. Iliid., p. 203, pi. cxii, figs. 2, 3. 



— Thetironia minob, F. StoUczha. PaliEont. Indica, Cret. Fauna S. luilia, 



vol. iii, p. 158. 

 1884. Thetis hiinok, 0. Weertli. Die Fauna des Ncoc-oni. iin Teutoburg. Walde 



(FaUvout. Abhaudl., vol. ii), p. 41, pi. ix, 

 figs, 5, 6. 

 1898. — LiEViGATA, E. G. Skeat and V. Madsett. Jur. Neoc. and Gault 



Boidders in Deuiiiark, 

 p. 176, pi. vi, figs. 7— 9. 



• Part of this rib is seen in a figure given by Pictet and Cainpiche, ' Terr. Cret. Ste. Croix ' 

 ("Mater. Pal. Suisse," ser. 4, 18GG), pi. cxxi, fig. 7a. 



