348 A Monograph of the Species of Pisidium. 
Arr. XXIIl.— A Monograph of the Species of Pisidium, 
found in the United States of North America, with Fig- 
ures. By Tempue Prime. 
In the preparation of this Monograph, I have taken pains 
to obtain specimens of the different species from the original 
describers, and thanks to kind assistance have mostly been 
successful, as also to compare specimens from different loca- 
lities. - 
In the description of the shells I have generally adhered to 
the text of the author, and have altered it but where I con- 
sidered it absolutely necessary. 
_ The figures are all magnified twice, with the exception of 
those of the P. ventricosum, P. ferrugineum, P. rotundatum, 
and P. Kurtzii, which are magnified four times. 
PISIDIUM Pfeiffer. 
SYNONYMY. 
Concha Linn. Tellina Linn. Cardium Poli. Chama Geoflroy. 
Musculus fluviatilis Lister. — Pectrunculus fluviatilis Lister. 
Spherium Scopoli. Cornea Megerle. Pisum Megerle. 
Corneocyclas Ferussac. Pera Leach. Englesia Leach. 
Gallileja da Costa. Physemoda Rafinesque. 
Under what family this genus ought to be placed has as 
yet not been decidedly settled by naturalists. Pfeiffer and 
Held place it among the Cardiums ; La Marck and Philipp! 
among the Conques fluviatiles, and Nilsson, Menke, Kleberg, 
Sowerby, Gray and Des Hayes alongside of the Cyclas and 
Pfeiffer is undoubtedly the first who separated this genus 
from Cyclas, but owing to his confounding the posterior and - 
anterior extremities of the shells in his description, his te- 
marks have not been as clear as they might have been. - 
