102 
beaks at the anterior fourth, elevated and in- 
clined forwards ; surface covered with numer- 
ous lines on the posterior portion, radiating 
from the umbones ; on the anterior portion, and 
extending nearly to the middle of the basal 
margin, are several large, sharp ribs, decussated 
by the lines of growth; teeth, two in each 
valve, one large and one small one in the 
right valve, and one large tooth in the left 
valve, deeply cleft, and a thin elevated one, 
pointing toward the margin, generally detached 
in removing the animal from the shell. Length, 
two inches; breadth, three-fifths; height, 
four-fifths. 
This shell, at first sight, resembles a Pholas, 
but is distinguished from it by its articulated 
hinge and byits not gaping widely. It is 
abundant on all our shores from Massachusetts 
Bay to Florida and the Gulf of Mexico; local 
and more rare from Massachusetts Bay to 
Prince Edwards Island. Its habitat in Rhode 
Island is from high to low-water mark in hard 
clay, peat, old logs, etc. 
Petricola dactylus, Say, not of Sowerby, Hanley 
and others, was described as a distinct species 
in Say’s American Conch., 1834, DeKay’s 
Nat. Hist. of N. Y., Binney’s Gould second 
edition, 1870. It is described as being broader 
and shorter, the ribs less prominent and the 
radiating lines more numerous. It is also said 
to inhabit deeper water. It is now acknowl- 
edged by Conchologists to be merely a variation* 
from the normal form of P. pholadiformis. 
FAMILY VENERID~. 
This is a very large family, consisting of 
many genera and species, a most elaborate clas- 
sification of which is found in Deshayes’ Cata- 
logue of the British Museum. The species are 
found in all seas, generally in shallow water ; 
they are strong and beautifully colored. ‘They 
made their first appearance in the Oolite and 
are now at their maximum of development. 
Four sub-families are recognized. 
SUB-FAMILY VENERIN. 
There are three genera, two of which are 
represented in Rhode Island. 
THE ~ CONCHOLOGISTS’ > EXCHANGE. 
Genus Venus, Linn., 1758, 
There are 176 species living, and 200 
fossil, Of the eleven sub-genera, into which 
the genera is divided, one is represented in 
Rhode Island by a single species. 
166.— Venus ( Crasstvenus) mercenaria, Linn, 
Syns : 
Venus mercenaria, Linn. 
Mercenaria mercenaria, Chenu, Dall. 
cs violacea, Schum, Stimp., Adams, 
Desh. 
cf notata, Desh, 
Venus notata, Say, Gld., Phil., Delay. 
“  preeparea, Say, Hanley, Desh. 
*  obliqua, Anton. 
Crassivenus mercenaria, Perkins. 
Shell large, thick and solid; surface chalky 
white with no epidermis ; in young specimens 
the surface is covered with sharply defined con- 
centric ridges ; as the shell grows older these 
become gradually obliterated until the surface 
is almost entirely smooth ; interior pure white, 
except at the margin where it is usually of 
a beautiful purple color. Length, four inches ; 
height, three; breadth, two. 
An extremely abundant species in Rhode 
Island, living at and below low-water mark in 
sand or mud, and known by the name of 
Quahog or Round Clam, It is an article of 
food, and from New York southwards it almost 
entirely takes the place of the common long 
clam, ‘The purple border of the inside of the 
valves was used by the Indians for the manu- 
facture of wampum, or circulating medium for 
money; the white wampum was made from 
the axis of Fulgur carica and Sycotypus canali- 
culatus. 
Linneeus, in Syst. Nat. ed xii. p. 1131, 1767, 
named this species, mercenaria, Schumacher, 
in 1817, separated from the genus Venus, a new 
genus which he called mercenaria, and as our 
species falls in that group, its name becomes 
mercenaria mercenaria. According to the rules 
of the British Association, specific names should 
not be made generic, On this account Dr. 
a eae 
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