154 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. 37. 
This shell is of no economic importance, but is interesting on ac- 
count of its boring habit and the singular form of the incrustation 
from which its subgeneric name was derived. The allied L. attenuata 
Deshayes, which also occurs on this coast, is distinguished by having 
the prolongations of its incrustation soegemnalls flat and oppor 
like a duck’s bill, instead of alternate. 
ARCA (ANADARA) GRANDIS Broderip and Sowerby. 
Plate 25, figs. 9, 10. 
Arca grandis BropErip and SowERBy, Zool. Journ., vol. 4, 1829, p. 365.—REEVE, 
Conch. Iconica, Arca, 1844, pl. 1, fig. 4 
Pata de Burro. From the oyster banks of Matapalo, near Capon, and at Huaquilla, 
on the northern border of Peru. A large coarse form eaten by fishermen. 
Distribution.—From Magdalena Bay, Lower California, south to 
Peru. Common in the mud about mangrove roots. 
Shell large, heavy, white, covered with a strong smooth dark oliva- 
ceous periostracum; obliquely subquadrangular, with strong radiat- 
ing rounded ribs crenulated only near the anterior end of the shell. 
The name applied by the Tumbes fishermen to this heavy coarse 
bivalve is the same which in the south they give to the univalve 
Concholepas. 
ARCA (SCAPHARCA) TUBERCULOSA Sowerby. 
Plate 27, fig. 4. 
Arca tuberculosa SowERBY, Proc. Zool. Soc. of London for 1833, p. 19.—REEvE, 
Conch. Iconica, Arca, 1844, pl. 3, fig. 18. 
Concha prieta. Mouth of the river Tumbes, and near Capon, from the muddy 
floor of mangrove swamps. Among the first phenomena to catch one’s attention on 
entering the mangrove swamps is a sound, heard repeatedly on every side, as of 
nuts falling into the water or the soft mud. Tracing the sound with some care, it is 
found to come from the watery hollows in the mud occupied by the concha prieta, 
and is presumably made by the sudden closing of its valves under water by the mol- 
lusk. This species, though inferior to some other shellfish of the region, is the one 
most commonly eaten. 
Distribution.—From Cedros Island, west coast of Lower California, 
in mangrove swamps and muddy places, south to Peru. 
Shell oval, turgid, oblique, the hinge line subauriculate, with numer- 
ous radiating ribs, armed, especially in front, with scattered tuber- 
cles; surface covered with a dense, pilose periostracum in life, the 
shell beneath white and porcellanous; ligamental area narrow, 
umbones adjacent. 
This very common shell somewhat resembles A. secticostata Reeve, 
of the Florida coast. 
