554 MOLLUSCA. 
PISIDIUM. 
Pisidium, C. Pfeiffer, Syst. Anordn. u. Beschr. d. deutsch. Land- u. Wasser-Schneck. i. p. 123 (1821). 
Pisum (nec Megerle, 1811), Gray, P. Z. S. 1847, p. 184. 
Musculium (nec Link, 1807), H. & A. Adams, Gen. Recent Moll. ii. p. 451 (¢f. p. 660, 1858). 
Fore part of the shell (before the summits) distinctly longer than the hinder part, 
which bears the ligament. Two small cardinal teeth in the left valve, two (or one) in 
the right. No pallial sinus. Of small size, more or less triangular or globular. ‘Two 
very short siphons. 
Generally distributed. 
1. Pisidium abditum. (Tab. XLIII. figg. 5, 5a, 6.) 
Pisidium abditum, Haldeman, Proc. Acad. Phil. i. p. 53 (1841)*; Deshayes, Catal. Conch. Brit. 
Mus. ii. p. 281°; Prime, Monogr. Am. Corbiculade, p. 68, fig. 72 (magnified) *. 
Pisidium retusum, Prime, P. Z. 8. 1860, p. 322°; Catal. Rec. Species Fam. Corbiculade, in Am. 
Journ. Conchol. v. 2, p. 172 (1869) °. 
“Testa minuta, ovata, subrhomboidea, complanata, ineequilaterali, tumida, postice rotundata, tenuiter striata, 
epidermide corneo-flava vestita, umbonibus tumidis.” (Prime *.) 
a. Long. 34, alt. 33, diam. 27 millim. Vertices in 3 long. 
b. ” 53; ” 4:6, 2) 3:3 ” 9 tr 9 
a. Measurements given by Prime for P. abditum; 6. Specimen from Cartago. 
Hab. Honpvuras (coll. Cuming *). 
CenTRaL Costa Rica: San José, 1135 metres (Biolley); Rio Torres, below San 
Francisco; San Francisco, 1100 metres (Pittier); Cartago (Aug. Miiller). 
Also recorded from San Rafael, New Mexico [Dall, Proc. U.S. Nat. Mus. xix. p. 370 
(1896)]. 
Prime first * described the specimens from Honduras as a separate species, but after- 
wards, in 1865 ?, he unites them with P. abditum, without distinguishing the Honduras 
examples even as a “variety”; but, in 18695, he again maintains the latter as distinct. 
It is unnecessary to quote here the full synonymy of P. abditum, as it refers to 
specimens from North-American localities only, extending northwards as far as Lake 
Superior, New England, and Montreal. The above diagnosis is taken from Prime’s 
description of the Honduras examples; but as no measurements are given, they must 
be compiled from the general descriptions of the North-American P. abditum. Clessin 
remarks that this species is the American analogue of P. fossarinum ( fontinale, OC. Pfr., 
part.), which is rather widely distributed in Northern and Middle Europe. 
The Berlin Museum has received from the dealer, Aug. Miiller, some specimens of 
a Pisidiwm, labelled ‘“‘Cartago, Costa Rica,” which agree very well with Prime’s 
description and figure, except in their larger size (see measurements 8). 
it may be remarked that the figure given by Clessin, in Martini and Chemnitz, 
Conch.-Cab. ed. 2, Cycladeen, t. 6. figg. 7-9, differs in its outlines considerably from 
