The position of the dorsal scars within the beak cavities is different, in the examples of the pseudo Nephronaias seen by the writer. Nephronaias (s. s.) possesses an (accessory?) adductor scar attached to the frontal portion of the cardinal teeth, which is either absent or obsoletely marked in the second assemblage. For this latter group the writer, therefore, proposes to use the generic term of Actinonaias Crosse and Fischer, 1893, type U. sapotalensis Lea. The female of this species has been described by Dr. Ortmann (1912). Actinonaias embraces, besides the type, medellinus, gundlachi, (accepting Simpson’s interpretation of this latter species), and others.
Psoronaias, Crosse and Fischer (1893). This group of remarkable shells, embracing crocodilarum, psoricus, semigranosus, etc., was provisionally treated by Simpson as a group of Elliptio, but their remarkable sculpturing, and the deep beak cavities of some of their species, led him to observe that it was possible that the group should, after all, be placed in Quadrula.
I follow von Martens, in giving generic rank as above to the group. The type is Unio psoricus. To this genus we are enabled to add a species hitherto undescribed, under the name of
Psoronaias kuxensis, n. sp., PI. VII, figs. 1, 2.
Shell small, compressed, rough, brown, biangular. Length 50, height 30, diam. 17 mm.
Shell hyperbolically rounded before, the extreme frontal point below the centre. Dorsum slightly arched, descending behind the ligament to the widely biangular posterior; the upper angle of which is midway the height, the lower angle very little above the base, which is nearly straight. The beaks are small, low, acute, approximate; and apparently, concentrically ridged. Epidermis dark brown (olivaceous and obsoletely rayed in the young), rough, the lines of growth numerous and well impressed. The discs are covered with fine pustulations, more pustular in front, biradially linear behind. The post ridge is low, but distinctly double, making the shell biangulate behind. The teeth are double in the left valve, single in the right. The cardinals are deeply sulcate and stout. Laterals slightly curved or nearly straight, separated by an interdentum. Nacre purple, beak cavities rather deep. Dorsal scars numerous, extending in a row from above the centre of the cavity down and forward upon the base of the cardinal teeth. Three well impressed muscle scars in front, two behind, the latter almost confluent. Habitat, Kux Creek, Chama, Guatemala. Collected by Mr. A. A. Hinkley, Feb. 6, 1917. A few dead specimens were obtained on the bank of the Isaibha River (Chama) of which the Kux Creek is a tributary. Type in Academy Natural Sciences. Cotypes in collection of A. A. Hinkley, the author and U. S. N. Museum.
I place this species in Psoronaias Crosse and Fischer, type U. psoricus, because of its evident relationship to crocodilarum, and distinctus, differing mainly from the latter in size and degree of inflation, being much inferior in both respects to distinctus.
ON THE RATE OF GROWTH OF POND UNIOS.
BY L. S. FRIERSON.
During the latter part of March 1916, the writer, for the purpose of constructing a fish pond, excavated a barrow-pit near the bank of a small creek, about ten feet wide, and at the time nearly dry. The barrow-pit was perhaps one hundred feet long, fifty feet wide and three feet deep. Early in April, 1916, the pit became full of water, overflowing from the adjacent creek, and together with two subsequent overflows, supplemented with seepage from the newly constructed fish pond, the pit remained more or less full of water, until May 25, 1917, when it was drained by a ditch into the nearby creek. From the dried bottom of this pit some thirty Unios were picked up by the writer. Ten of these were Unio tetralasmus Say, and the rest were T. texasensis Lea. All the specimens were of remarkably uniform size and appearance. The texasensis being about one and a half inches, and the tetralasmus two and a half inches long. Exact dimensions of a texasensis: length 43, height 24, diam. 16 mm.; of tetralasmus 75, 40, and 25 mm.