PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 401 
Leda unca Gould. 
Proce. Bost. Soc. Nat. Hist., viii, p. 282, 1862.—Otia Conch., p. 239 (=? Leda 
acuta Conrad, described as fossil). 
Many of our specimens are much larger than the shells described by 
Gould and Conrad. Our larger specimens are 13"" long, 8™™ broad. 
This shell is rather strong and thick, oval, swollen, rounded anteriorly, 
but posteriorly narrowed to an acute, short, angular beak, at the base of 
which there is a slight incurvature of the ventral edge. The nearly 
straight posterior dorsal edge slopes regularly to the beak, and is some- 
what compressed or keeled. The whole surface is covered with numer- 
ous prominent, regular, rounded, concentric ribs, separated by deep 
grooves of about the same width. On the posterior dorsal area these 
ribs are smaller, and are often nearly obsolete close to the edge. 
Taken in considerable numbers, alive and dead, at many of the sta- 
tions, both south of Martha’s Vineyard and south of Newport, R. I., in 
$5 to 155 fathoms, especially at stations 871, 873, 874, and 876. 
This species appears to be allied to L. Messanensis Cant. (= DL. acumi- 
nata Jeft.), from deep water in the Mediterranean. 
Leda pernula (Miiller). 
G. O. Sars, op. cit., p. 35, pl. 5, fig. 1 a-d. 
A specimen that appears to be a typical example of this speceies was 
dredged by us in 1877, off Halifax, in 59 fathoms. It has a smooth, 
lustrous, yellowish-green epidermis. The concentric grooves are irreg- 
ular and mostly obsolete, except anteriorly, where they are fine and 
close. The form is similar to that of ZL. tenwisulcata. Length, 23"; 
height, 10". 
Yoldia frigidia Torell. 
Spitz. Moll., p. 148, pl. 1, fig. 3, 1859.—G. O. Sars, Moll. Reg. Arct. Norv., p. 39, 
pl. 4; figs. 11a, b. 
This species occurred at station 894. It had not previously been ob- 
tained off the New England coast, but had been dredged in the Gulf of 
Saint Lawrence, by Whiteaves, in 200 fathoms. 
Arca glacialis Gray. 
G. O. Sars, op. cit., p. 43, pl. 4, figs. 1 a-c.—Verrill, Trans. Conn. Acad., v, 
pl. 44, fig. 5. 
This species has been dredged in numerous localities by the various 
dredging parties of the United States Fish Commission, since 1872, in 
the Bay of Fundy, Gulf of Maine, off Cape Cod, on George’s and Le 
Have Banks, and off Halifax, Nova Scotia, at various depths from 90 
to 430 fathoms; about 70 to 75 miles south of Martha’s Vineyard, in 
115 to 192 fathoms, and south of Newport, in 85 to 500 fathoms. It 
attaches itself to pebbles or gravel-stones by a small but strong ventral 
byssus. 
The shorter and more rounded form, known as Arca pectunculoides 
Scacchi, also occurs on our coast, as well as the deformed variety called 
var. septentrionalis by G.O. Sars. These appear to me to be mere vari- 
Proc. Nat. Mus. 80-——26 Jan. 10, 1881. 
