///. xhtll has (see fig. 21) by its very inequilateral form, at first glance some resem- 

 blance to the shell of the genus Crenella. Brown, but differs from it, as will hereafter appear, 

 in many essential points. It is in the largest specimens 5 millimetres long. 4 millimetres 

 high in the posterior part and 3 mill, in the anterior part, the thickness across both valves 

 together 3 mill. It is however .often smaller 34 mill. long. The xhripr is rhomboidal or 

 oval-trapezoidal, inflated, the posterior part somewhat compressed, the middle part extending 

 from the umbones to the posterior ventral corner (or diagonally) strongly convex. 



The anterior extremity is narrowly rounded, much lower (by nearly half) than the 

 posterior extremity which is vertically truncated and nearly straight, sometimes even slightly 

 incurved in the middle and with rounded corners. The dorsal and ventral borders are like- 

 wise nearly straight and parallel, or only slightly convex, and form a right angle with the 

 posterior border, but with rounded corners. The umbones lie quite in front, so that the anterior 

 extremity of the shell only advances a very little beyond them, and near to each other (sub- 

 contigui); they are (see fig. 24) 'strongly projecting forwards and inwards and somewhat spi- 

 rally involved.- There are no evident lunula. The right valve (fig. 25) is a very little larger 

 than the left (tig. 26) especially in the posterior part, so that both its dorsal and ventral 

 borders project a little beyond those of the left valve, and are curved a little outwards round 

 the latter (see fig. 21 & 23). 



In other respects the valves close accurately without gaping at any point and tin- 

 edges of the shell are everywhere smooth, not dentated. 



The shell is of tolerably thin and fragile consistency; whitish and somewhat trans- 

 parent, the exterior surface having only a slight or dull polish; the interior surface brighter, 

 with a weak mother-of-pearl-like iridescent lustre. 



Its outer surface, where there appears to be a very thin hyaline epidermis (covered 

 with strongly agglutinated fine grains of sand) which how r ever cannot be detached from the 

 shell, is, like that of the genus Lyonsia, radiately striped with fine raised smooth lines distant 

 from each other and in number 1G 20 running from the umbones to the edge. The intervals 

 between these radiating lines are extremely finely punctuated; and the points appear under 

 the microscope (fig. 29. 30) to be somewhat projecting small cylindrical or truncated conical 

 processes in radial rows, 68 rows in each interval. The lines of growth are extremely fine 

 and close, parallel with the edge and difficult to perceive. 



The interior surface of the shell is smooth, and shews (see fig. 25 A: 20) 2 roundish 

 or somewhat oval slight adductor-impressions, the anterior one very distinct and situated near 

 the middle of the anterior edge; the posterior one, less evident and situated close under 

 the posterior superior corner. The impression of the mantle-edge (the mantle-line) is simple 

 without any sinus. 



The khnjc is without teeth; the ligament internal, situated in a somewhat oblique 

 longitudinal groove near the dorsal edge of each valve, and containing, as in the Lyonsia, 

 a small opaque-white free movable ossicle (fig. 26. 27. 28 a) which lies longitudinally under 



