404 Gould's Descriptions of Shells 
and is covered by a thick chestnut colored epidermis, conspicuously 
corrugated at every part, in vertical wrinkles posteriorly, but in a 
somewhat divaricate manner anteriorly. The posterior superior 
portion gives rise to a byssoid fringe. 
Length three inches ; height and breadth half an inch. 
Found in indurated marly clay. Monterey. Maj. Rich. 
This species is so peculiar, and so entirely different from any 
other of the genus, as to require no comparison. 
AVICULA STERNA (Pl. XVI. Fig. 7.) 
Avicula sterna, Gould ; Proc. B. S. N. H., Nov. 1851. IV. 93. 
T. solidula, badia radiis pallidis ornata ; epidermide cinereá serratim fimbriatà 
induta; apicibus ad quadrantem anteriorem positis; alà semicirculari, modice 
obliquà; declivitate umbonali tumida, abrupta ; cauda longissimá, gracili; aurt- 
is magnis, radiatim striatis; fissuré byssali profundissima, acutangulari: 
margarita argentata, valdé iridescens; limbo fusco-marmorato. 
Shell rather solid, obliquely ovate, triangular, with a very long, 
slender, acuminate cauda, and well marked auricles: color dark 
mahogany brown with a few paler radiations; surface compactly 
striate, covered with a thin, dirty, cinereous epidermis, very deli- 
cately fringed with radiating series of minute processes, especially 
in the vicinity of the byssal notch and at margin. Beaks near the 
anterior fourth, prominent, oblique, the posterior umbonal slope 
tumid, and in the left valve limited by an obtuse groove, a sub- 
marginal angle along the cauda ; auricles radiately striate. Auricle 
of right valve trapezoidal, widening towards tip and obliquely tron- 
cate ; byssal notch deep and acutángular; auricle of left valve sub- 
triangular, as broad as long, the byssal fold prominent. Hinge wit 
a well marked cardinal and lateral tooth in left valve; the cardinal 
tooth in right valve double. Cavity of beaks very decided. Nacre 
silvery and highly iridescent; cretaceous margin like tortoise shell. 
ngth of hinge margin two and a half inches; -diameter of wing 
one and one fourth inches; breadth five eighths of an inch. 
bags Mazatlan ? Lieut. Green. Panama. Prof. Adams, Col. 
ewett. 
Has the general appearance of A. semi-sagitta, but the wing " 
less oblique and curves directly into the cauda without any $!nU** 
