POLYMESODA. 541 
Shell moderate in size. Three converging cardinal teeth in each valve ; anterior lateral teeth shorter than 
the posterior, not serrated. A small but distinct pallial sinus. Outlines triangular, obliquely cordiform 
or rounded (as in the genus Venus); inside often violet. Hinder orifices of the mantle prolonged into 
short siphons. 
Peculiar to America, ranging from Georgia and California to Bolivia. 
The presence of the pallial sinus may well be regarded as a generic character, 
more particularly as it coincides with the geographical distribution, all the species being 
American and the Cyrene-proper without pallial sinus belonging to the tropical 
regions of the Old World. R 
Polymesoda, evidently a shortened form of Poly-mes-odonta, with many middle teeth. 
The species of this genus may be arranged in three subdivisions, viz. :— 
I. Potymzsopa, s. str. See above, p. 540.—Shell very thick, rounded-triangular, concentrically 
ridged ; hinder part of the dorsal margin obliquely descending to the junction with the ventral 
margin, more or less swollen; summits moderate, close together. Type P. carolinensis, 
Bosc. Geographical distribution as stated above for the whole genus, but not known from 
the West Indian islands. 
II. Eorra, H. & A. Adams, Gen. Moll. ii. p. 651 (1858) : Cyrena (Anomala), Deshayes, P.Z.S. 
1854, p. 21 (without definition) ; Cyrene anomale, b. testa tenuis, rotundata, cordiformis, 
Deshayes, Catal. Conchifera Brit. Mus. ii. p. 257. [Nec Anomala, Stephens (1830), 
Burmeister, Laporte.]—Shell comparatively thinner, very swollen, concentrically striated, 
with broad prominent summits, which are usually much decorticated ; general outline more 
obliquely cordiform than triangular, biangulated behind. Species peculiar to Central 
America and Colombia. 
III. Cyrenocapsa, Fischer, Ann. Lyc. N. York, 1872, p. 194; Journ. de Conch. xx. p. 404 
(1874) ; Fischer & Crosse, Miss Scient. Mex. ii. p. 18 (1894).—Shell thin, smaller in size, 
oval, much longer than high, nearly smooth. Hinge-plate narrow. Pallial sinus indistinct. 
Species chiefly estuarine. Type C. salmacida, Morel. Yucatan, Cuba, Bahama Islands, 
and Florida. 
The species said to be estuarine will be dealt with later among the brackish-water 
forms, but for convenience (as in Neritina) they are included here in the Comparative 
Table ; it is, however, quite possible that some of those supposed to live in fresh water 
will prove to be estuarine. 
1. Polymesoda olivacea. (Tab. XLII. figg. 6, 6 a.) 
Cyrena fontainei, Philippi, in Zeitschr. f. Malak. 1851, p. 70'; Deshayes, Catal. Conchifera Brit. 
Mus. ii. p. 253° (nec d’Orbigny, 1844). 
Cyrena olivacea, Carpenter, Catal. Mazatlan Shells, p. 114 (1857) *; Mollusks of W. N. America, 
p. 27 (1872) *; Prime, Monogr. Am. Corbiculade, p. 17, fig. 12 (1865)°; Clessin, in 
Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Cycladeen, p. 114, t. 16. figg. 5, 6 (1877) °; 
Sowerby, in Reeve’s Conch. Icon. xx., Cyrena, t. 9. fig. 32"; Fisch, & Crosse, Miss. Scient. 
Mex., Moll. ii. p. 634, t. 70. figg. 5, 5a, 6°. 
