ANODONTA. §29 
g- Long. 46; alt. vert. 27, ale 33, diam. 14 millim. Vertices in ¢ long. 
A. 6 686; - 235 x 2l, 10. - i, 
ah. Selected from examples collected by Richardson: a-d. Apparently full-grown; a and c. More elongated ; 
6 and d. More elevated and triangular specimens ; e-h. Apparently young shells. 
Young specimens appear to be generally more compressed, less produced behind, and not so distinctly 
truncated, with the ventral margin more arcuated, and the summits with broadly undulated sculpture. 
Hab. Centra Mexico: Lake of Chapala, State of Jalisco (Biart & Diguet*? ; 
Richardson) ; Mexico, without nearer indication of locality 1-8, 
This species, living in a lake which drains into the Pacific, resembles in many respects 
A, exilior, Lea, from Eastern Mexico, but it is comparatively more elevated. 
Fischer and Crosse’s figures of A. chapalensis agree so well with that of Kiister’s 
A. coarctata, that there can be little doubt as to their identity. 
The typical specimens of dA. chapalensis described and figured by the French 
authors*® seem to be much worn: in their general outlines they are very like the 
full-grown shells collected by Richardson, but their size is that of the young ones; 
they also have the anterior part comparatively shorter and more abrupt than in any of 
the others before me. Nevertheless, I refrain from treating Richardson’s specimens as . 
belonging to a different species, as we know that, even in Europe, the shells living in 
large lakes show a great individual variability. Fischer and Crosse’s examples are 
probably full-grown, young shells being so fragile and brittle that they cannot be 
worn so much without being broken; I presume that they were found in another part 
of the same lake, and that this species reaches, when full-grown, a different size in 
different parts of the lake. I figure several specimens of various ages, in order to show 
that the younger shells are more orbicular and the older ones more lengthened, 
especially the very old example (fig. 7). | 
2. Anodonta richardsoni, sp.n. (Tab. XLI. figg. 1, La, 2, 2a.) 
Triangularly oval, comparatively high, and rather convex, anteriorly lower and rounded, dorsal margin behind 
the summits somewhat ascending and then without an angle, sloping down convexly to the hinder 
extremity, which is also rounded, without any distinct angle; ventral margin regularly arcuated. 
Summits not prominent, in 4 of the length, much worn; in the young shell with feeble undulated 
sculpture. Periostracum grey, with close concentric strie and frequent broader ridges, indicating steps 
of growth, of the same colour. Inside pale dull bluish, often somewhat reddish in the centre. 
. Long. 59; alt. vert. 36, ale 41, diam. 23 millim. Vertices in 3 long. 
R 
b. ”? 45 ; ” 28, ” 32, ” 17 ” ” 3 ” 
c. ” 41 ; oe 25, 29 28, ” 153 ” ” 3°99 
d. 39 363 ; oP) 23, 2? 25, ” 12 29 ” 3 ” 
I. 
a. Full-grown ; ’-d. Younger specimens from the Ameca Rive 
Hab. N.W. Mexico: Rio Ameca, State of Jalisco (fichardson, Dec. 1884). 
Somewhat like A. coarctata, but more rounded in all respects and dull, without lustre. 
A. tehuantepecensis, Crosse & Fisch., is also nearly allied, but its dorsal margin is more 
convex, while in A. richardsoni it is nearly straight for a short distance, and then 
descends distinctly downwards for a greater distance. 
BIOL. CENTR.-AMER., Terr. and Fluviat. Mollusca, August 1900. 67 
