TRIGONOARCA. 47 
Greensand, the apparent differences between the two forms being explained by 
the imperfect state of preservation of the specimens from the Upper Greensand of 
Devizes. A comparison of a number of examples from these and other localities 
confirms the view of d’Orbigny. 
The specimens from the Gault are, on the average, of larger size than those 
from the Blackdown Greensand, and, owing to their different mode of preserva- 
tion, differ a little in appearance from the latter. The record of this species from 
the Lower Greensand of Kent, given by Morris, is apparently erroneous. 
Types.—The types of both Arca carinata and Cucullea costellata are in the 
British Museum. 
Distribution.—Greensand of Blackdown (zones viii to xii, especially x) and 
Haldon. Upper Greensand of Devizes and Ventnor. Gault of Black Ven. Zones 
of Acanthoceras mammilatum and Hoplites interruptus at Okeford Fitzpaine. Gault 
(zones 1, 11, vili—x) of Folkestone. Chalk Marl (zone of Schlenhachia varians) of 
Ventnor and Folkestone. 
Genus—Triconoaroa, 7. A. Conrad, 1863. 
[‘ Proc. Acad. Nat. Sci. Philad.’ (1862), 1863, p. 289; ibid., 1872, p. 54; ‘Amer. Journ. Conch.,’ 
vol. iii (1867), p. 9; Appendix to W. C. Kerr’s ‘Rep. Geol. Surv. N. Carolina, vol. i, 1875 
(Raleigh) ; figured, ‘Journ. Acad. Nat. Sci. Philad.,’ ser. 2, vol. iv (1860), p. 281, pl. xlvii, 
fig. 20.] 
Triconoarca Passyana (d’Orbigny), 1844. Plate VIII, figs. 9 a—c, 10a, b. 
1832. CucunLea cartnata, A. Passy (non Sowerby). Descript. Géolog. de la 
Seine-Infér., p. 8 (of expl. of plates), pl. xiv, 
fies. 11, 12. 
1844. Anca Passyana, A. d’Orbigny. Pal. Frang. Terr. Crét., vol. iii, p. 241, 
pl. eeexxvii, figs. 1, 2. 
1850. — — = Prodr. de Pal., vol. ii, p. 164. 
1866. — _ F. J. Pictet and G. Campiche. Foss. du Terr. Crét. de 
Ste. Croix (Matér. Pal. Suisse, ser. 4), pt. 38, p. 475. 
1871. — _- F. Stoliczka. Palwont. Indica, Cret. Fauna S. India, 
vol. ii, p. 843 (Zrigonoarca). 
1896. — _ tLicertensis, A. J. Jukes-Browne and W. Hill. Quart. Journ. Geol. 
Soce., vol. lii, p. 153 (from Chard). 
21897. Cucutnrma Passyana, R. Leonhard. Die Fauna der Kreidef. in Ober- 
schles. (Paleeontographica, vol. xliv), p. 51. 
Description.—Shell oblong, more or less elongate, convex, inequilateral, rather 
short anteriorly. Ventral margin nearly straight, placed a little obliquely ; 
