118 EOCENE MOLLUSCA. 
attached to it. The nearest approach to this in outward form is JV. 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 WV. disu/cata, but that shell has a 
smooth margin. 
17. Nucuua sruinis, J. Sowerdy. Tab. XVIII, fig. 11 a—e. 
Nucuta simiLis. J. Sow. Min. Conch., t. 192, fig. 10, 1819. 
— — Morris. Catal. Brit. Foss., p. 218, 1854. 
— TrIGoNA. J. Sow. Min. Conch., t. 192, fig. 5, 1819. 
ARCA NUCLEUS. Solander, in Brand. Foss. Hanton., p. 40, t. 8, fig. 101, 1766. 
Spec. Char. NV. testd ovato-trigond, crassd, sub-compressd, levigatd aut obsoleté radiata, 
concentrice irregulariter lineata ; siphoni-regione brevissimd, truncata, ad basim eversd, 
sub-acuminata ; lunuld 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, 3ths 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. ¢rigona, 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, V. mixta, ‘ An. sans vert. du Bassin de Par.,’ t. 1, p. 819, pl. 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 WV. 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. 
