LEPTON. . 99 



like sculpture, tlie surface appearing under a lens of small 

 power to be punctured all over, but under the microscope 

 to be most crowdedly set with the most minute tubercles 

 imaginable. The ventral margin is retuse in the middle, 

 and rises with nearly equal arcuation at either extremity. 

 The sides are very nearly equal, and are rounded at their 

 extremities, both the front and hinder edges being well 

 arcuated, and their chief swell rather below than above 

 the middle ; from this point the edges incline inwards, so 

 that the dorsal margin, which on the shorter side declines 

 convexly and but slightly, whilst it almost rises on the 

 longer and very slightly broader side, is manifestly shorter 

 than the lower one. The upper portion of the extreme 

 margin of the longer side is rather more oblique and 

 straight than the corresponding portion of the shorter 

 one ; hence, the former side projects rather the more 

 below, and the latter is rather the more (but not quite) 

 symmetrically rounded. The beaks, which scarcely ap- 

 pear above the dorsal line, are very acute, and hardly 

 lean to either side. There is neither lunule nor umbonal 

 ridge, and the dorsal edges do not bend inward on either 

 side. 



The interior is glossy, and adorned with fine radiating 

 lines ; the edges are quite plain. The hinge of the right 

 valve consists of two short nearly parallel teeth-like 

 laminae, on either side of, and immediately adjacent to, 

 a broad triangular central excision ; of which the anterior 

 set are rather the more abbreviated. The left hinge ex- 

 hibits a narrow apical tooth, which curves slightly for- 

 ward, behind which lies a subtriangular excision of the 

 hinge-margin, and on either side an adjacent sublateral 

 lamina ; of which the anterior, which is decidedly the 

 shorter, and occasionally somewhat resembles an oblique 



