544 MOLLUSCA. 
Triangular, cordiform, dull yellowish-green ; hinder area somewhat flattened, limited by a very obtuse keel ; 
_ ventral margin strongly arcuated, slightly sinuated behind. Concentric ridges close together, flat, very 
unequal and some of them compound, somewhat wrinkled, particularly near the ventral margin. Hinder 
extremity subtruncated, there being a very obtuse angle near the hinder end of the lateral teeth and 
another much below at the junction with the ventral margin ; ventral margin arcuated, slightly sinuated 
at its hinder end. Pallial sinus narrow, much ascending. Inside pale violet, with. more intense 
violet zones. 
a. Long. 54, alt. 45, diam. 30 millim. Vertices in 3 long. 
dB 64, 62, ,, 52, ,, 384 ,, » 2,4, 
2) 
a. Specimen in Dunker’s collection ; 6. One in Paetel’s collection. 
Hab. N.W. Mexico: Mazatlan (eigen 3-8). | 
Central America, without nearer indication of locality (Deshayes ). 
Also occurs in Lower California, at La Paz (Major Rich ®). 
A younger specimen, long. 35, alt. 30, diam. 19 millim., differs from the full-grown 
ones in having a more shining surface, and in the absence of any sinuation at the 
hinder end of the ventral margin; the hinder part of the shell is comparatively 
somewhat shorter, the summits, therefore, in two-fifths of the whole length. 
In the example figured by Clessin © the ventral margin is more unequally arcuated, 
the sinus of the hinder part thus being more conspicuous than usual in this species. 
Obs.—P. fontaine, VOrb. [Voy. Am. mér. v. p. 469, t. 83. figg. 16, 17 (1835-43) |, 
from Guayaquil, is more shining, with more regular, stronger, very close concentric 
ridges, the hinder extremity somewhat more produced and uni-angulated, the pallial 
sinus shorter and more oblique. 
2. Polymesoda solida. (Tab. XLII. figg. 8, 8a, 9, 9a.) 
Cyrena solida, Philippi, Abbild. neuer Conch. i. p. 78, t. 1. fig. 9 (Feb. 1846) ‘; Deshayes, Catal. 
Conchifera Brit. Mus. ii. p. 254°; Prime, Monogr. Am. Corbiculade, p. 14, fig. 8°; 
Clessin, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Cycladeen, p. 109, t.18. figg. 7-9°; 
Tate, in Am. Journ. of Conch. v. p. 160 (1870) ° [nec C. solida, Dunker, Ueber die nord- 
deutschen Walderthone, p. 23 (1843-44); Monogr. der norddeutschen Wealdenbildungen, 
p. dl, t. 10. fig. 20 (1846, not 1834 as stated by Fischer’); nec C. solida, Sowerby, in 
Reeve’s Conch. Icon. xx., Cyrena, t. 18. fig. 104, from Australia (1876) ]. 
Cyrena nicaraguana, Prime, Am. Journ. of Conch. v. p. 146 (1869)°; Fisch. & Crosse, Miss. 
Scient. Mex., Moll. ii. p. 640, t. 64. figg. 2 (?), 2a, 37. 
Polymesoda, sp. indet., Morch, in Malak. Blatt. vii. p. 193 (1860-61) *. 
Unequilaterally triangular, very swollen, fore side shortly rounded, hinder side much descending above, bluntly 
angulated in its lower part, without a distinct ridge limiting the posterior area ; ventral margin strongly 
arcuated. Outside somewhat silky in appearance, shining, dark greenish-brown, near the ventral margin 
often light green; concentric ridges strong, most of them regular, a few bifurcating or anastomosing. 
Inside more or less violet or pale reddish, adductor impressions darker violet ; pallial sinus diagonally 
oblique, rather short, triangular. 
a. Long. 40, alt. 38, diam. 28 millim. Vertices in 4 long. 
b. 4» 42, 4 B88, 4, 274, 
c« 4 41, , 37, 4, 29 , 
a, Figure given by Philippi; 6, c. Specimens from Nicaragua in Dunker’s collection. 
99 39 
enine Coles 
” 9 
