BIVALVIA. 113 



in similis, which is its nearest relative. It bears a strong resemblance to N. margaritacea 

 of Nyst, ' Coq. Foss. Belg./ p. 229, pi. xvii, fig. 9, a, b, but the dorsal margin of that 

 figure is rather more curved. N. margaritacea, Goldf., 'Petr. Germ.,' vol. i, p. 158. t. 

 125, fig. 21, a — d, also much resembles it. 



Some specimens in Mr. Edwards's cabinet from the same locality have attached to them 

 the MS. names of quadrans, Tab. XVIII, fig. 8, a, b, and planiuscula, same plate, fig. 9 ; 

 they differ in outward form, as those names indicate, but I think they are varieties of the 

 above. It is possible they may prove to be distinct. 



9. Nucula Headonensis, Forbes, MS. Tab. XVIII, fig. 3, a, b. 



Nucula Headonensis. Morris. Mem. Geol. Surv., p. 156, pi. 6, figs. 12, 12, a, b, 1856. 

 — — Id. Catal. Brit. Foss., p. 218, 1854. 



Spec. Char. " Testa ovato-lransversd, depressd, lavi, latere antico brevi, subproducto, 

 margine arcuato, postice angusto, margine ventrali subarcuato, intus cremdato ; lunula 

 prominuld, sulco perspicuo circumdatd." 



" An ovately transverse and somewhat depressed shell, with the anterior margin short 

 and slightly produced, the posterior extremity narrowed, the ventral margin arched and 

 internally very finely crenulated ; the lunule prominent, and surrounded by a conspicuous 

 furrow." — Morris. 



Length, jjths of an inch. 



Localities. Colwell Bay, Headon Hill {Morris and Edwards), Hordwell (S. Wood). 



This species has been separated from similis by the late Edward Forbes ; it has also 

 been considered by Mr. Morris as distinct, and I readily acquiesce in the separation. 

 Specimens are not particularly rare, and are found often in a very perfect state of preser- 

 vation. It somewhat resembles in outline N. Dixoni, but it is a smaller species ; thesiphonal 

 region is shorter, and the basal extremity on that side is more rounded, and it appears also 

 to be a thicker shell. In the 'Catal. Brit. Foss.,' p. 218, 1854, Nucula similis, Wood, is 

 given as a synonym to this species, but I have not been able to find the reference. 



10. Nucula lissa, Edwards, MS. Tab. XVIII, fig. 4, a, b. 



Spec. Char. N. testa elongato-ovatd, ienui, lesvi, glabra, compressiusculd, ineequilaterali; 

 pedi-regione latiore, ovatd; siphoni-regione oblique truncatd, subrostrata ; marginibus dorsali 

 et ventrali convexis ; lunula lanceolato-elongatd, depressd ; ano cordiformi ; dentibus ad apicem 

 gradatim minutissirnis ; marginibus integris. 



Shell elongately ovate, thin, smooth, and glossy, slightly compressed, inequilateral ; 

 pedal-region the wider and ovately rounded ; siphonal region obliquely truncated \ dorsal 



