66 



Perhaps = Unio A(hincuf^, Sowerby. "Mineral Conehology," Vol. VI., page 190, 

 Plate DXCV., figs. 2, 2. 



Shell convex, elongated ; outline ovately triangular or ovately wedge 

 shaped ; anterior side short and rounded ; posterior side long and bluntly 

 pointed at the extremity ; length greatly in excess of the height. Beaks 

 moderately large, compressed at almost a right angle to the sides of the 

 valves, incurved, pointing slightly tbrwards, and placed at a distance of 

 about one-fourth the length from the anterior end. Lunule none ; ven- 

 tral margin straight, or a little concave behind the middle, more pro- 

 minent under the beaks, and curving up regularly in front. Posterior 

 side very variable in shape, usually moderately elongated and sub- 

 triangular, but sometimes much more lengthened and narrowly attenuate. 

 In the typical form the hinge margin slopes convexl}^ and rather rapidly 

 downwards from behind the beaks, and .the posterior extremity still 

 more abruptly so, the latter being bluntly pointed below. The normal 

 contour of this part of the shell is elongately subtriangular, two of the 

 sides being gently convex and the central angle rounded. 



In an exceptionally lengthened variety the posterior side is narrowly 

 attenuated and wedge shaped ; the hinge border descends obliquely in 

 an almost unbroken straight line, and the posterior extremity is narrowly 

 rounded. 



A blunt ridge extends downwards and backwards from the beaks to 

 the posterior end of the base, and in so doing separates an obliquely 

 flattened area from the rest of the shell. The ligament is external and 

 proportionately rather narrow. 



The surface is marked b}^ coarse and irregular lines of growth ; the 

 beaks, which are often much eroded, are undulately corrugated when 

 perfect. 



Out of thirty specimens collected by Mr. Eichardson, twenty-nine have 

 both valves, with the test, preserved, and the ligamenf even is visible in 

 some. In every case, however, the posterior extremity, which is the 

 thinnest and therefore weakest jjart of the shell, is broken off. Some- 

 times the surface of the test is partly covered by a thin film of pyrites. 



The only detached valve collected, a right valve, having beeri soaked 

 a long time in water, an attempt was made to remove the matrix thus 

 softened and to expose the hinge teeth. Although the rather thick test 

 broke in pieces during the operation, it was found that besides the 

 ordinary cardinal teeth, there was a longitudinal groove in the right 

 valve, for the reception of a corresponding lateral and laminar tooth in 



