NUCULA. 19 



Sub-genus — Acila, H. and A. Adams, 1858. 

 (' Genera of Recent Mollusca,' vol. ii, p. 545.) 



Nucula (Acila) bivirgata, Sowerby, 1836. Plate III, figs. 1, 2a — c, 3, 4, ha — c, 



6—12. 



1836. Nucula bivirgata, J. de C. Sowerby. Trans. Geol. Soc, ser. 2, vol. iv, 



p. 335, pi. xi, fig. 8. 

 1844. — — A.cVOrbigny. Pal. Fran?. Terr. Cret., vol. iii, p. 176. 



pi. ccciii, figs. 1—7. 

 1844. — ornatissima, d'Orbigny. Ibid., p. 175, pi. cccii, figs. 9—12. 

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



1854. — ornatissima, Morris. Ibid., p. 217. 



1855. — bivirgata, G. Cotteau. Moll. Poss. de l'Yonne, p. 84. 



1866. — —F.J. Pictet and G. Campiche. Poss. du Terr. Cret. 



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

 p. 418. 

 1866. — ornatissima, Pictet and Campiche. Ibid., p. 418. 

 1875. — bivirgata, A. J. Juices- Broivne. Quart. Journ. Geol. Soc., vol. 



xxxi, p. 299, pi. xv, figs. 4—8. 

 1884. — — J. S. Gardner. Quart. Journ. Geol. Soc, vol. xl, 



p. 132, pi. iii, figs. 12—14; pi. iv, 

 figs. 20—22. 



Description. — Shell oval, convex, angular posteriorly, rounded anteriorly. 

 Antero-dorsal margin slightly convex, postero-dorsal concave. Ventral margin 

 evenly curved. Lunule cordate, depressed, ornamented with transverse ribs; 

 central part elevated. Escutcheon narrow, elongate, not sharply limited, orna- 

 mented with ribs. Surface of shell ornamented with numerous fine radiating ribs 

 crossed by delicate lamellae; the ribs diverge at an acute angle from a line 

 extending from the umbo in an antero-ventral direction. In some cases the ribs 

 diverge from two lines near together, formiug an inverted W, thus AA. New 

 ribs may become intercalated between the others, especially near the margins of 

 the valves of larger specimens. Margins of valves crenulated. Teeth : posterior 

 about six, anterior about twelve. Adductor impressions distinct, rounded, near 

 the margins ; the anterior larger than the posterior. 



Measurements : 





(i) 



(2) 



(3) 



(4) 



(5) 



(6) 



(7) 



Length 



. 19 



16 



14 



15 



18-5 



23 



18 mm. 



Height 



. 13 



12 



10 



12 



13 



15-5 



13 „ 



Antero-dorsal margin 



. 16 



14 



11 



11-5 



16 



20 



15 „ 



Postero-dorsal margin 



. 9 



8 



7 



7 



8 



11 



8 „ 



Thickness 



. — 



— 



— 



— 



11 



— 



12-5 „ 



