ACEPHALA. AAB 
PERNA TorvA (Gould). 
T. rudis, crassa, inequivalvis, concentricé lamellata, cinerea, obliqua, 
trapezoidea ; latere byssali dilatato, semicirculari ; sinu byssali pro- 
fundo; apice adunco; latere postico rectilineart, obliquo, producto ; 
margine cardinali obliquo: area cardinalis triangularis, sulcis obliquis 
ad sex arata: margarita ex argenteo-purpurascens ; limbo lato, retro 
dilatato. 
Perna torva, Goutp; Proceed. Boston Soc. Nat. Hist., ii. 311. 
Nov. 1850. Expedition Shells, 92. 
SHELL rude, rather solid, elongated, somewhat trapezoidal, oblique: 
posterior margin oblique, nearly straight: anterior base abruptly round- 
ed: posterior edge and base broadly rounded: colour pale drab: hinge 
margin short, oblique, widening anteriorly: teeth, about six, slender. 
Nacre silvery white: calcareous limbus broad, especially behind : 
ground-colour of the exterior ashy, but clouded dusky: nacreous por- 
tion scarcely oblique: byssus fissure small, intruding but little into 
the shell, nearly equal in both valves: beaks acute. . 
Length an inch and a half; height an inch and three-fourths ; 
breadth three-eighths of an inch. 
Inhabits the Sandwich Islands. 
The obliquity of this species, and the small number of its cardinal 
sulci, are among its most prominent characters. 
Figures 558, 558 a, outside and inside of the shell. 
Perna NANA (Gould). 
T. parva, crassa, subquadrata, adunca, lamellosa, extus nigra ; margine 
ventrali sinuato; angulo infero-postertort rotundato ; latere postico 
sinuato: area cardinalis lata, 6-sulcata: fissura byssalis ampla, tro- 
gonalis: margarita plumbaginea ; limbo corticali angusto, pallido. 
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