438 MOLLUSCA. 
portion, and forms a shorter curve at the byssal margin; so that the 
general curve of the whole shell is forwards. Right valve most con- 
vex; byssal notch a broad deep groove running to the apex, over which 
a small auricle with a simple margin partially projects; in the left 
valve the auricle is still less conspicuous, and there is a large rounded 
tuberosity answering to the notch in the opposite valve. Nacre faint 
greenish, its edge smoky, beyond which the cortical substance projects 
a considerable distance, lurid and dusky green radiated ; no conspi- 
cuous hinge teeth. 
Figure 555, side view of the shell. 
The above description is here introduced, because no detailed cha- 
racters are to be found in any preceding work; and it seems quite 
necessary as a standard of comparison for other species, several of 
which have been confounded with this. It differs from others in the 
absence of well-marked auricles, and in the peculiar manner in which 
the ventral margin is curved, much after the form of the point of 
some old-fashioned table-knives. No other species seems to attain to 
so large a size. 
In describing the species of Avicula and Perna, the form of the 
nacred interior, which is constant, is to guide us, rather than the ex- 
terior form, which is often rendered quite diverse by the cretaceous 
margin which extends beyond it. In the following descriptions this 
rule has been observed. 
AVICULA NIGRA (Gould). 
Testa mutica, inequivalvis, ecaudata, edentula, fusca, antrorsum lobata ; 
valva dextra convexiuscula, posticé integer, antice profundé emargi- 
nata; auricula triangulari, crenulata: valva sinistra convezxior, pos- 
tice profunde emarginata: margarita ex argenteo cerulescens ; limbo 
lato, fusco. 
Avicula nigra, GouLp; Proceed. Boston Soc. Nat. Hist., iii. 309. 
Nov. 1850. Expedition Shells, 90. 
Suevy small, thin dolabriform, siightly antrorsely curved, height and 

