Anatomy.[[7]]
All these shells have the same, and an extremely characteristic and unique structure of the soft parts, so that there is not the slightest question that they belong together. I have examined the soft parts of some 200 specimens in the field, and over three dozens have been preserved in alcohol, and have been examined at leisure in the laboratory. They include representatives of the three main varieties, and of practically all of the individual variations.
Gravid females have been found on the following dates: May 11, ’13; May 15, ’13; May 16, ’13; May 20, ’13; May 20, ’14; May 22, ’14; May 25, ’14; July 5, ’13; July 9, ’13; July 10, ’13; July 13, ’13; July 14, ’13. Glochidia have been observed on May 20, ’14 (immature), and July 14, ’13. Thus this species evidently is a summer breeder (tachytictic).
The soft parts are those of the genus Fusconaia: the supraanal is separated from the anal by a very short mantle-connection, which is absent (or torn?) in rare cases. Inner lamina of inner gills free from abdominal sac. All four gills are marsupial. Placentae well developed and subcylindrical.
Branchial opening with well developed papillae, anal with distinct, but small papillae. Palpi subfalciform, posterior margins connected at base only.
While thus the Fusconaia structure is typically developed, this species is quite unique in its color. This concerns chiefly the color of the gonads, eggs, and placentae.
The soft parts are often uniformly pale, whitish, but may shade to orange, and the orange is most prominent on foot, adductors, and mantle-margin; but the paler tints prevail, and often the orange is replaced by yellowish or brown. The gills are pale, but are generally suffused with blackish. The gonads are brown to red, mostly of a peculiar dull lavender color in the female, and the latter color, or purplish brown, is the prevailing color of the eggs and placentae. The charged gills become thus rather dark purple, or purple-brown, shading sometimes to dull red or blackish, in other cases to brownish, brownish pink, brick-red, or even pale brown. These are very peculiar tints, by which this species is easily recognized in the field: four marsupial gills of this blackish-purple color are not known in any other Nayad.
Glochidia have been found only in specimens belonging to the headwaters variety (barnesiana bigbyensis). They are subelliptical, slightly higher than long, L. 0.15, H. 0.16 mm.
Although a true Fusconaia, this species (or group of forms) stands isolated within the genus, in characters of the shell as well as in the soft parts. It differs from the species of the subrotunda-group (ind. ebena, pilaris etc.) very markedly by its smaller size and by the very shallow beak cavities. The forms of the undulata-group (incl. flava, and the cuneolus- and corforms) have generally also somewhat deeper beak cavities, and the shell has a more or less distinct posterior ridge, with a flattening or a shallow groove in front of it, characters which are missing in the barnesiana-group. As has been pointed out, in the latter group, the color of eggs and placentae is remarkable: in all other forms of Fusconaia, this varies from white to bright red.
I introduce here another species, in order to show that the barnesiana-type is also represented outside of the Cumberland-Tennessee drainage, namely in the Ozarks.