BIVALVIA. 83 



resemble the parallel lines upon A. Iieterodonta, Desh. ('An. sans vert, dn Bass, de Par.,' 

 p. 906, pi. 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, A. obliquaria, 

 Desh. (id., p. 893, pi. 67, figs. 8 — 10, JO bis), but is larger, shorter, and not quite so 

 oblique. It is separated from A. depressa, as well by difference in outline as by the 

 apparent difference in the hinge-area. 



6. Arca eximia, Edwards, MS. Tab. XV, fig. 3. 



Spec. Char. A. testa elongatd, oblique sub-quadrangidari, valde incequilaterali, sub- 

 depressd, radiatim costulatd et concentrice decussatd, punctata ; umbonibus prominentibus, 

 distantibus ; area connexus Icevigatd ; dentibus mediants minimis ; siphoni-regione productd. 



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, |ths 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, A. punctifera, Desh. (p. 202, pi. 32, figs. 13, 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 Erench 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. A. exornata, Desh. (' An. sans vert, du 

 Bass, de Par.,' p. 889, pi. 69, figs. 1 — 3), as also A. intersecta, figs. 25 — 27, resemble it 

 in some characters. 



12 



