DIODONTA. 285 



Our native variety is of a produced ovate or slightly 

 trigonal form, more or less strong, almost opaque, and sub- 

 ventrlcose or even slightly inflated. It is decidedly in- 

 equilateral, almost devoid of lustre, and when fresh, usually 

 tinted externally with brownish saffron. The surface, 

 which is apt to be indented in various places, is wrought 

 with concentric, elevated, somewhat undulated laminar 

 striae, (which are occasionally almost obsolete,) varying in 

 their approximation to each other, and in the distinctness 

 or obscurity of the very minute radiating lines which de- 

 cussate their interstices. The ventral margin, whose out- 

 line is often wavy, is moderately convex ; the dorsal edges 

 are very slightly retuse on either side of the beaks, then 

 convex and rather sloping in front, and almost straight 

 and decidedly sloping behind. The anterior side, which 

 is conspicuously the shorter, is broadly rounded at its ex- 

 tremity ; the posterior termination is bluntly acuminated, 

 and very slightly subrostrated. The umbonal fold is broad 

 and rather obtuse, the ventral flexure is tolerably evident. 

 The beaks are small, acute, and almost directly inflected, 

 scarcely leaning to either side ; the umbones are sufficiently 

 prominent ; the ligament is rather large, and moderately 

 projecting. The dorsal lips bend considerably inward in 

 front of the beaks. The interior is either of a dirty white 

 or stained with the external colouring. The hinge is not 

 provided with any distinct lateral teeth ; there are two 

 moderately sized divergent primary teeth in the right valve, 

 which become much more elevated towards the Internal 

 edge, and a single broad and very large bipartite subre- 

 flected central tooth in the left valve. There appears to be 

 a very thin oblique laminar denticle behind this tooth, but 

 it is so extremely caducous, that it is rarely present. The 

 palleal sinus is extremely large. The length of rather a 



