THETIRONIA. 169 



more inequilateral than the Shanklin form. After comparing a considerable number 

 of specimens I find that those from Atherfield are, as a rule, slightly less convex ; 

 but the more inequilateral character is not constant; specimens of the same size 

 are commonly quite as nearly equilateral, but the inequilateral character becomes 

 more marked with age. Since larger forms are more common at Atherfield than 

 at Shanklin, One may at first sight get the impression that there is a real difference 

 in the inequilateral character. I have not seen any specimen from Atherfield so 

 inequilateral as the form figured by Pictet and Campiche appears to be, and I think 

 it is probable that their specimen was somewhat distorted by crushing. 



T. geneuensis, Pictet and Roux, 1 is distinguished from T. minor by the height and 

 length being equal, by its more nearly equilateral form, and by a difference in the 

 curvature of the rib in front of the angular part. 



Remarks. — Thetlronia has been recorded by Phillips and Judd from the 

 Speeton Clay, but I have not seen any examples from that deposit. In some 

 collections from Speeton specimens of Ct/prina have been identified as Thetlronia. 



Type. — Fig. 6, Lower Greensand of Shanklin ; in the British Museum. The 

 original of fig. 5 from near Lyme Regis has not been seen. 



Distribution. — Lower Greensand : Crackers of Atherfield ; also recorded by 

 Fitton from the Perna-hed and Beds vi, viii, ix, x, xiii, and xiv between Ather- 

 field and Blackgang Chine. Ferruginous Sands of Shanklin. Atherfield Beds of 

 East Shalford. Hythe Beds of Hythe. Sandgate Beds of Sandgate, etc. Folke- 

 stone Beds of Folkestone. 



Thetiuonia laevigata {Sowerby), 1818. Plate XXVI, figs. 0—14. 



1818. Corbula laevigata, J". Sowerby. Min. Conch., vol. iii, p. 1-1, pi. ccix, 



figs. 1, 2. 



1826. Thetis major, J. de C. Sowerbij. Ibid., vol. vi, p. 20, pi. dxiii, figs. 1 — 4. 



1829. M. J. L. Befrance. Diet. Sci. Nat., vol. liv, p. 276. 



1850. Ukicardium l^vigatum, A. d'Orbigny. Prodr. de Pal., vol. ii, p. 163. 



1854. Thetis l^vigata, J. Morris. Cat. Brit. Foss., ed. 2, p. 227. 

 — major, Morris. Ibid., ed. 2, p. 227. 



1862. J. G. Ghenu. Manuel de Conchyl., vol. ii, p. 90, fig. 405. 



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



(Mater. Pal. Suisse, ser. 4), pp. 209, 

 210, pi. cxii, fig. 5. 



1 Pictet and Roux, ' Moll. Foss. Gres verts de Geneve' (1852), p. 420, pi. xxx, fig. 2. Pictet and 

 Campiche, ' Terr. Cret. de Ste. Croix ' (1865;, p. 206, pi. cxii, fig. 7. Barrois records T. genevensis from 

 the Upper Greensand of Lulworth, but I have not seen any specimens ; see Barrois, ' Terr. Cret. Super, 

 de 1'Angleterre,' etc. (1876), p. 90. 



