NUCULA. 19 
Sub-genus—Aciia, H. and A, Adams, 1858. 
(‘ Genera of Recent Mollusca,’ vol. ii, p. 545.) 
Nvcuna (Acina) Brvircata, Sowerby, 1836. Plate III, figs. 1, 2a—e, 3, 4, 5a—e, 
6—12. 
1836. Nuvcvta pivineata, J. de C. Sowerby. Trans. Geol. Soc., ser. 2, vol. iv, 
p- 335, pl. xi, fig. 8. 
1844. — ~~ A. @Orbigny. Pal. Franc. Terr. Crét., vol. iii, p. 176. 
pl. eceiil, figs. 1—7. 
1844. — ORNATISSIMA, d’Orbigny. Ibid., p. 175, pl. eccii, figs. JI—12. 
1854. — BivinGata, J. Morris. Cat. Brit. Foss., ed. 2, p. 217. 
1854. _ ORNATISSIMA, Morris. Ibid., p. 217. 
1855. — BIVIRGATA, G. Cotteau. Moll. Foss. de l’Yonne, p. 84. 
1866. — -- F. J. Pictet and G. Campiche. Foss. du Terr. Crét. 
de Ste. Croix (Matér. Pal. Suisse, ser. 4), pt. 3, 
p. 418. 
1866, _- oRNATISSIMA, Pictet and Campiche. Ibid., p. 418. 
1875. — BIVIRGATA, A. J. Jukes-Browne. Quart. Journ. Geol. Soe., vol. 
XXXxi, p. 299, pl. xv, figs. 4—8. 
1884. — — J. 8. Gardner. Quart. Journ. Geol. Soe., vol. x1, 
p. 182, pl. iii, figs. 12—14.; pl. 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 lamelle; 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, forming an inverted W, thus M. 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 : 
GQ @) (@) @ (5) (6) (7) 
Length : F . 19 16 14 215 18°5 23 18 mm. 
Height : : oat elo LOM AZ 13 16°5:" 13) 4; 
Antero-dorsal margnm . 16 14 11 115 16 20 Le 
Postero-dorsal margin . 9 8 7 7 8 11 tate oh 
Thickness : : LS 11 — 195) 
