THETIRONIA. 107 



yet they are occasionally absent. Other differences are seen in the very thin and 

 punctate shell, and in the internal rib. 



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

 any other lamellibranch. But in some Jurassic and Cretaceous 1 species of Proto- 

 cardia a rib, either single or A-shaped, is found at the inner boundary of the 

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

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

 the anterior adductor. 



It seems, therefore, that although Thetironia resembles the Cardiidae in several 

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

 that family. 



Thetironia minor (Sowerby), 182G. Plate XXV, figs. 15 a— c; Plate XXVI, 



figs. 1—8. 



1822. Venus, G. Mantel!. Fuss. S. Downs, p. 73. 



1826. Thetis minor, J. de C. Sowerby. Min. Conch., vol. vi, p. 21, pi. dxiii, 



figs. 6 (? 5). 

 1829. M. J. L. Def ranee. Diet. Sci. Nat., vol. liv, p. 275. 



1841. — Sowerbii, F. A. Burner. Die Versteiu. d. nord-deutsch. Kreidegeb. 



p. 72 (partim). 

 18-15. var. a minor, var. p. major. E. Forbes. Quart. Journ. 



Greol. Soc, vol. i, p. 212. 

 181(3. ljevigata, A. d'Orbigny. Pal. Franc. Terr. Cret., vol. iii, p. 452, 



pi. ccclxxxvii, figs. 1 — 3. 

 1832-53. minor, G. P. Deshayes. Traite Element, Conchyl., vol. i, p. 575, 



pi. xxii, figs. 3, 4. 

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



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

 p. 318). 

 1865. F. J. Pietet and G. Campiche. Foss. Terr. Cret. Ste. Croix 



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

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

 1870. laevigata, Pietet and Campiche. Ibid., p. 203, pi. cxii, figs. 2, 3. 



Thetironia minor, F. Stoliczka. Palaeont. Indica, Cret, Fauna S. India, 



vol. iii, p. 158. 

 1884. Thetis minor, 0. Weerth. Die Fauna des Neocom. im Teutoburg. Walde 



(Palaeont. Abbandl., vol. ii), p. 41, pi. ix, 

 figs, 5, 6. 

 1898. laevigata, E. G. Skeat and V. Madsen. Jur. Neoc. and Cault 



Boulders in Denmark, 

 p. 176, pi. vi, figs. 7 — 9. 



1 Part of this rib is seen in a figure given bj Pietet and Campiche, ' Terr. Cret. Ste. Croix 

 ("Mater. Pal. Suisse," ser. 4, 1866), pi. exxi, fig. 7a. 



