290 OSTREAD^. 



flesh-colour, diversified in the more characteristic specimens 

 with pale or whitish markings, which vary infinitely in 

 number, size, shape, and conspicuousness. This painting is 

 generally replaced in the lower valves by a white or paler 

 tint, usually, however, displaying the more characteristic 

 painting upon the umbones and costal interstices. A 

 peculiar dull pearly- white gloss distinguishes the interior of 

 the more mature individuals ; the younger shells, from their 

 thinness, exhibit more or less of the external hue and 

 pattern. The valves are more or less round, rather thin, 

 moderately and nearly equally convex, and rayed with 

 from four to ten (five or seven are the most usual) unarmed 

 rounded and unequal ribs, which vary much in elevation, 

 expansion, and distance from each other, but are commonly 

 more abrupt and angular above, more dilated and depressed 

 below. In the thinner and more ordinary form (of this a 

 live but perfectly colourless individual is in the cabinet of 

 Mr. Barlee) the surface is smooth, and the shape, owing 

 to the bulging out of the lower posterior portion, is inequi- 

 lateral and oblique ; the ribs are usually six or seven in 

 number, and owing to the gentle declination of their sides 

 give an undulating appearance to the exterior. The dorsal 

 slopes are but trifling, and the general contour nearly 

 circular ; the auricles are comparatively subequal, rather 

 larffe, inclined to be rectangular, and smooth or striated ; 

 the sinus is short, shallow, and scarcely angular. Certain 

 individuals of this form exhibit under the microscope a 

 sculpture analogous to that of tigrinus, but still more 

 minute, with the radiating lineoles less divergent, and the 

 concentric wrinkles more conspicuous. We regard these, 

 however, as exceptional. 



The variety Dumasii is so aberrant, that it was only 

 after protracted scrutiny and the comparison of many 



