AAA MOLLUSCA. 
The peculiar magnesian external coating, radiated with dark purple, 
the golden nacre, and the porcelain-like marbled limbus, make this a 
strongly marked species. The name first given is preoccupied. 
Figures 549, 549 a, exterior and interior of the shell; 549 8, dorsal 
view. 
PERNA ARGILLACEA (Gould). 
T. solidula, mutica, compressa, elongata, anticé dilatata, postice subrecta, 
ex luteo alutacea, hic illic nigricans ; apice adunco: fissura byssalhs, 
angusta, lanceolata: intus livescens: area cardinals curta, obliqué 
triangularts, sulcis obliquis ad octo arata: fovea apicalis profunda. 
Perna argillacea, Goutv ; Proceed. Boston Soc. Nat. Hist., i. 311. 
Dec. 1850. Expedition Shells, 91. 
SHELL solid, smooth, compressed; pale drab-colour, clouded with 
dusky, especially at periphery ; elongated, somewhat triangular, trun- 
cated at tip; surface laminated, and somewhat squamate ; hinge short, 
a little oblique; posterior margin a little concave: anterior a little 
concave at the byssal fissure, then dilating, becomes continuous with 
the slightly rounded base. Nacre drab marbled with livid, thin: 
calcareous limbus broad, very thin, of nearly the same colour: outline 
of nacre angular: hinge area triangular, grooves eight or ten, long: 
byssus-opening formed by an inflection mostly in the right valve: 
apicial pit deep. 
Length two and a half inches; height three inches; width three- 
eighths of an inch. 
Habitat uncertain. 
The resemblance to the common P. ephippium of the West Indies 
is very great. It differs chiefly in the clay-coloured exterior, and in 
having the byssal instead of the opposite side dilated; the muscular 
scar 1s also smaller. 
Figure 559, outside of the shell; 559 a, inside of the shell. 
