118 EOCENE MOLLUSC A. 



attached to it. The nearest approach to this in outward form is N. sericea, Tab. XIX, 

 fig. 3, but that is a smaller shell, with the dorsal margin less convex and the ventral 

 margin less curved than in this species ; this shell is more ovate or less wedge-shaped, 

 with the terminal portion of the siphoni-lateral margin more pointed, and the siphonal 

 region slightly compressed. It resembles in outline N. bisulcata, but that shell has a 

 smooth margin. 



17. jNucula similis, J. Sowerby. Tab. XVIII, fig. 11 a — c. 



Nucula similis. J. Sow. Min. Conch., t. 192, fig. 10, 1819. 



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



— trigona. /. Sow. Min. Conch., t. 192, fig. 5, 1819. 



Arca nucleus. Solander, in Brand. Foss. Hanton., p. 40, t. 8, fig. 101, 1766. 



Spec. Char. N. testa ovato- trigona, crassd, sub-compressd, laevigata aut obsolete radiatd, 

 concentrice irregulariter lineatd ; sip/ioni-regione brevissimd, truncatd, ad basim eversd, 

 sub-acuminatd ; lunula anoque angulo obtuso ; dentibus numerosis ; marginibus crenulatis. 



Shell ovately trigonal, thick, slightly compressed, smooth, or indistinctly radiated, and 

 irregularly furrowed concentrically ; siphonal region short, truncate, with the basal 

 termination everted and somewhat pointed ; lunule and corselet indistinctly defined ; 

 denticles numerous, rather thin and compressed ; margin crenulated. 



Length, 1 inch ; height, ^ths of an inch. 



Locality. Barton. 



Considerable difficulty exists with regard to this species, of which numerous specimens 

 are found at Barton. It is, I believe, the true Arca nucleus of Solander, a specimen from 

 his collection being still in the British Museum. JV. trigona, Sowerby, is probably only the 

 young, or, at most, a variety of this species ; and as it is so marked upon Mr. Edwards's 

 tablet, I am pleased to find he is of the same opinion. There is a French shell figured by 

 M. Deshayes, N. mixta, ' An. sans vert, du Bassin de Par.,' t. 1, p. 819, pi. 64, figs. 1 — 4, 

 which very closely resembles it ; but he says the distinctions between the specimens 

 themselves are evident and permanent, and such as will justify specific separation. Our 

 shell is a handsome one, and is of considerable solidity, rather flat or compressed ; it is 

 readily distinguished from all other species excepting N. mixta. The distinguishing 

 character is an extension or pointedness at the extreme basal portion of the siphonal 

 region, and the hinge also is somewhat peculiar, often presenting broad teeth near the 

 umbo. 



