Measurements : 





(i) 



Length 





. 33 



Heigh b 





. 21 



Thickness . 





. 14 



Affinities. — The 



larger 



forms 



(5) 



(6) 



(V) (8) 



22-5 



21 



19 18 mm. 



13 



15 



13-5 12-5 „ 



— 



11 



9 9-5 „ 



22 CRETACEOUS LAMELLIBRANCHIA. 



(2) (3) (4) 



29 27 26 

 20 17 15 

 12 — — 



of this species, described by Michelin as 

 N. capsseformis, and subsequently named N. Derancei by Price, have been regarded 

 by some authors as distinct from Mantell's N. ovata, but practically the only 

 difference is in size : in France a perfect gradation has been traced between 

 these two types ; and a similar passage, although it is stated by Gardner not 

 to exist, can be seen in English specimens. In England the larger forms are 

 recorded by Price from zones i and ii only of the Lower Gault. N. ovata 

 is related to N. obtusa, Sowerby ; the points which distinguish the two are 

 given on p. 23. 



Remarks. — The variations in the relative proportions of the length, height, 

 and thickness are fairly great, as will be seen from the table of measurements. 

 The antero-dorsal border may be parallel to the ventral, or may slope more or 

 less obliquely to it. The posterior extremity is commonly somewhat angular, 

 but may be rounded. 



Types. — I have not been able to trace Mantell's specimens. The examples 

 figured by Gardner and referred to above (except, perhaps, figs. 28 to 30) are in 

 the British Museum. 



Distribution. — Through all the zones (except vii) of the Gault at Folkestone; 

 Gault of Black Ven ; Cambridge Greensand (derived) ; Upper Greensand of 

 Devizes. 



Nucula obtusa, Soiverby, 1836. Plate IY, figs. 2 a, b, 3, 4 a — d. 



1836. Ntjcula obtusa, J. de C. Soicerly. Trans. Greol. Soc, ser. 2, vol. iv, 



p. 342, pi. xvii, fig. 11. 

 1850. — — A.d'Orlirjny. Prodr. de Pal., vol. ii, p. 163. 



1854. — J. Morris. Cat. Brit. Foss., ed. 2, p. 217. 



? 1SGG. — — F. J. Fictet and G. Oampiche. Foss. du Terr. Cret. de 



Ste. Croix (Mater. Pal. Suisse, ser. 4), pt. 3, pp. 415, 

 418, pi. cxxix, fig. 15. 

 1884. — — J. 8. Gardner. Quart. Journ. Geol. Soc., vol. xl, p. 126, 



pi. iv, tigs. 1, 2. 



Nou 1844. — —A. d'Orligny. Pal. Franc. Terr. Cret,, vol. iii, p. 163, 



pi. ccc, figs. 1 — 5 (named N. planata 

 on pi. ccc). 



