BIVALVIA. 83 
resemble the parallel lines upon 4. heterodonta, Desh. (‘An. sans vert. du Bass. de Par.,’ 
p- 906, pl. 67, figs. 22—25), but the lines upon our shell appear to be in the area for 
connexus, and not upon the dental margin ; this I am unable correctly to ascertain. Our 
shell is very inequilateral, and the siphonal region is not only longer, but larger and 
higher ; the radiations are a little wider or further apart on the larger or broader portion 
of the shell than upon the pedal region. This shell resembles, in outline, 4. od/iquaria, 
Desh. (id., p. 893, pl. 67, figs. 8—10, 10 bis), but is larger, shorter, and not quite so 
oblique. It is separated from 4. depressa, as well by difference in outline as by the 
apparent difference in the hinge-area. 
6. Arca ExImIA, Hdwards, MS. Tab. XV, fig. 3. 
Spec. Char. A. testa elongata, oblique sub-quadrangulari, valdé inequilaterali, sub- 
depressd, radiatim costulatdé et concentrice decussatd, punctatd ; umbonibus prominentibus, 
distantibus ; area connewtis levigatd ; dentibus medianis minimis ; siphoni-regione product. 
Shell elongate, oblique, irregularly quandrangular, very inequilateral, somewhat de- 
pressed, punctated, covered with radiating lines or riblets, crossed by distinct lines of 
growth ; beaks rather prominent ; area of the connector rather narrow and smooth; teeth 
small in the middle of the dental area; siphonal region produced. 
Length, sths of an inch, 
Locality, Brook (Edwards). 
A single specimen of the left valve of a species which appears to be quite distinct, 
and to which is attached the above MS. name, is in the cabinet of Mr. Edwards ; it is 
not quite perfect, and so closely attached to the matrix that I am unable to see the 
interior. Its nearest relative is, I think, .4. punctifera, Desh. (p. 202, pl. 32, figs. 18, 14), 
but it differs from that species in several characters. The English shell appears to be more 
elongated and more inequilateral, and to have the area for connexus larger and broader 
than that described in the French shell, in which it is characterized as being so small and 
narrow as to bring the umbones almost close together, giving thereby a very small extent 
for the marginal separation of the valves. Our shell has the surface regularly rayed, with 
rather narrow and rounded single lines, which are decussated by broad and prominent 
lines of growth, leaving between each a deep depression or puncture ; and where the rays 
are narrowest these punctures are, of course, most numerous. ‘The area for connexus is 
broad, flat, concave, and smooth, and widest over the pedal region ; the denticles are close 
and numerous, inclining towards the extremity of the hinge-line. I am unable to see if 
the internal edges of the margin be denticulated. 4. exornata, Desh. (‘ An. sans vert. du 
Bass. de Par.,’ p. 889, pl. 69, figs. 1—3), as also A. cnterseeta, figs. 25-—27, resemble it 
in some characters. 
