TllETiKO.NlA. IG'J 



more inequiliitcral 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 tlian 

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

 in the iue(juilateral character. I have not seen any specimen iVum Atlierfield so 

 inequilateral as the form figured by Pictet and Campielie appears to Ije, and I tliiid< 

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



T. geneceiisis, Pictet and Roux,^ is distinguished from T. minor by the heiglit and 

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

 curvature of the rib in front of the angular part. 



BemarliS. — Thetlronia has been recorded by I'hillips and .ludd fi'oni the 

 Speeton Clay, ])ut I have not seen any examples from tliat deposit. In some 

 collections from Speeton specimens of Gijprlnci have been identified as Tlictironia. 



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

 original of fig. 5 from near Lj'me Regis has not been seen. 



Distribution. — Lower Greensand: Crackers of Atherfield; also recorded l)y 

 Fitton from the Per;/ (t-bed and Beds vi, viii, ix, x, xiii, and xiv l)et\veen Ather- 

 field and Blackgang Chine. Ferruginous Sands of Sliaidvlin. Atlierfield Beds of 

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

 stone Beds of Folkestone. 



Theiikonia l.kvi.;.vt.v {Sna-n-}„i), 1818. Phite XXVI, figs. 9—1-1. 



1818. CoRiiULA L.EViGATA, /. Soicvrhi/. Mill. CoUC'll., vul. ill, 1'. 1 I. [il. CL'ix, 



figs. 1, 2. 

 182G. Thetis major, /. de C. Suirerhij. Ibid., vol. vi, p. 20, pi. dxiii, fif,'s. 1 — I. 

 1829. — ~ M.J. L. Di/raiice. Diet. Sci. Nat., vol. liv, p. 27G. 



1850. Unicaedium ljevigatum, A. d'Orhignij. Prodr. de Pal., vol. ii, p. 1(33. 

 1854. Thetis l.*:vigata, /. Morris. Cat. Brit. Foss., ed. 2, p. 227. 

 — — MAJOR, Afurris. Ibid., cd. 2, p. 227. 



18(52. — — J. a. Cluiiii. Mauuel de Coiichyl., vol. ii, p. 90, fig. 405. 



18G5. — — F.J. Fictetaiul G. Citiiijilche. Foss. Terr. Cret. St t-. Croi.\ 



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

 210, pi. cxii, fig. 5. 



1 Pictet aiid Rous, ' Moll. Foss. Grc-s verts de Geucve' (1852), p. 420, pi. xxx, fig. 2. Pictet and 

 Campicbe, ' Terr. Cret. de Ste. Croix ' (18(55;, p. 20(5, pi. cxii, tig. 7. Barrois records T. (jciicvensis from 

 the Upper Greeusaud of Lulwortb, but I have iiot seen any specimens ; see Barrois, ' Terr. Cret. Sujht. 

 de I'Angleterre,' etc. (187G), p. 90. 



