50 TESTACEOUS MOLLUSCA. 
dorsal edge anteriorly sub-arcuated, posteriorly rounded, ventral 
rounded, anteriorly ventricose ; left valve anteriorly with radiating 
lines, a conspicuous keel near the anterior dorsal edge. 2..44. 
P. Ceytonica. Sow. Spec. Conc. t. 2. f.20,1, 2. Elongated, 
depressed, sub-flexuous, anteriorly beaked, anterior dorsal edge 
arcuated and recurved, posteriorly dilated; two strong teeth and a 
marginal lamina in the flat valve, a single arched one in the other. 
3..1,— Ceylon. 
P. Rapiata. Sow. Spec. Con. t. 2. f. 23, 4. Ovate, rather 
depressed, white with rufous linear rays on the flat valve ; anterior 
side longer, sub-truncated and its dorsal edge straight, posterior 
side small ; ventral margin rounded, 7,3%.—W. Columbia. 
P. Srriara. Quoy. Ast. t. 83. f.10. Ovate-triangular, rather 
solid, with transverse plait-like striee, depressed, posteriorly obtuse, 
anteriorly sub-angulated ; sub-equilateral; apex acute and emar- 
ginated.— New Zealand. 
TRIBE LITHOPHAGI. 
Shells which bore into clay and other substances, without accessory 
valves or sheath, and more or less gaping at their anterior side ; 
ligament external. 
SAXICAVA. 
Inequilateral, obtuse, and transverse ; the anterior and ventral 
margin gaping ; hingenearly without teeth ; ligament external. 
S. Rucosa.* Lam. 1.—Sow. G.— Reeve. t.50.—Myrixuvs. R. 
Lin.?—D., p. 304. (part.)\—W. t. 12. f. 9. Ovate, obtuse at each 
end, anterior side at least twice as long as the posterior, the ventral 
edge incurved in the middle; dirty white, strong, coarse, with 
transverse wrinkles and the epidermis yellowish ; beaks not ter- 
minal: if with two ridges on the anterior slope, the ridges very 
obscure. 3..4.—N. Europe.) 
S. Pooxapis.* Lam. 3.—Disrorta. Say.—Gould Mas. f. 
40.-—-Rucosa. Turt. Biv. t.2.f.10.—Mytivus. KR. D. p. 304. 
(part.)—Byssomya P.? BI. t. 80. bis. f.5.— List. t. 426. f. 267. 
Oblong, anteriorly more or less truncated, (sometimes produced 
and attenuated, as often dilated,) posteriorly obtuse and the beaks 
subterminal, ventral edge nearly straight and not incurved ; dirty 
white, strong, coarse, (the epidermis yellowish,) with concentric 

18. Gariicana. Lam. 2.is a var (fide Desh.) I doubt whether 
Pholadis is justly separated. 
