16 Mr. Swainson’s Description of 
than eight inches and a half; and its greatest breadth (from the 
ligamental to the basal margin,) five inches three-quarters. 
Its form is a broad oblong-oval; obtuse at both extremities; the 
anterior of which is broadest and sinuated, aud the posterior 
rounded, and so very short as to project only three quarters of an 
inch beyond the outer side of the cardinal teeth. The whole shell 
is remarkably flat but very thick in substance; and the umbones, 
which are unusually small have scarcely any convexity; the liga- 
mental margin is dilated, winged, and forming in its dilation nearly 
two equal sides; the horny part of the ligament itself, (with the in- 
ternal plate that supports it,) extends half way between the umbo 
and the extremity of the wing; the exterior colour of the epidermis, 
is dark brown, but the umbones (in this specimen) are decorticated 
for a considerable space around them. The sculpture of the an- 
terior part of the shell is very peculiar; it consists of four series of 
short oblique grooves, or of indented wrinkles, three of which are 
arranged in a direction with the umbonial slope *, the other is trans- 
verse; the first of these series consists in parallel grooves which 
cross the wing obliquely from left to right. The next is a range of 
broad and sinuated indentations, wide apart and having the same 
inclination as the former; the third range occupies the umbonial 
slope, and is formed by narrow sulcated grooves, placed nearly in 
a horizontal direction, and diminishing in length as they approach 
the umbones. The fourth and last consists of several transverse 
erooves situated near the basal margin, and the whole presents 
an appearance as if the shell had been indented, .in various di- 
rections, by some blunt instrument. 
The inside is pearly, white, tinged with flesh-coloured purple, 
and stained (as is frequently the case, in fluviatile bivalves) with 
olivaceous yellow spots; in a perfect state of the shell the colours, 
probably, would be more brilliant. 
The cardinal teeth are strong; deeply and irregularly striated 
and are obliquely transverse; in the right valve are two, and in 
the left valve, one; in each valve there is only one lateral tooth, a 
very unusual and discriminative character for the species; this 
*See Zoologi Ilust. 
