148 UNIONISE. 



slightly convex, and rises much more obliquely, and consi- 

 derably in front than behind. The anterior dorsal edge is 

 never retuse, but either arcuated or slightly convex ; it 

 runs almost parallel to the opposite outline, the attenuation 

 and occasional blunt subrostration of the hinder termina- 

 tion not extending far towards the beaks. These latter 

 are almost always eroded, displaying the olivaceous nacre 

 of the scarcely elevated umbones. The anterior side occu- 

 pies but little more than a fourth of the entire length, and 

 is rounded at its narrowed extremity, but not symmetri- 

 cally so, the declination of the upper portion of the more or 

 less arcuated upper edge not being so oblique as the slope 

 of the lower one ; just above the teeth there is a short sub- 

 retusion of the dorsal edge, but except in the young no 

 distinct lunular impression. The basal corner of the hinder 

 end is well rounded ; the upper posterior corner is often 

 obtusely angulated in the immature shell, but in the full- 

 grown one no angle exists, as the posterior edge forms an 

 almost uninterrupted curve with the elongated hinder 

 dorsal. There is no prominent umbonal fold, but a gra- 

 dually widening slight hollow runs from the umbones 

 adjacent to the dorsal edge, indenting the margin a little 

 at its termination. The ligament is long, but its projection 

 is very trifling. The internal nacre, which is of a bluish- 

 white, frequently flesh-coloured towards the centre, gene- 

 rally stained with irregular spots or markings of bronze or 

 green, and usually impressed near the middle with scat- 

 tered indented dots, does not extend to the margin, but 

 leaves a greenish border beyond it. The front muscular 

 scar is very profound ; the twin teeth of the left valve are 

 erect, strong, and subcorneal or pyramidal, the posterior 

 with four or five denticles upon its front surface, the an- 

 terior narrower, entire, and more or less pointed ; the sin- 



