Length of a middling sized specimen, four and a half inches, breadth, six and a quarter. It is often eighteen inches in circumference, round the border of the valves, with a diameter through the umbones of three inches. Inhabits the Upper Mississippi, from Prairie du Chien to Lake Pepin.
This fine shell, much the largest I have seen of the genus, was first sent by Mr. Schoolcraft, to the Lyceum, several years ago. So far as I am able to discover, it is undescribed, and a distinct and remarkable species. It may be known by its length being greater in proportion to its breadth than in the other American species, by the subrhomboidal form of the posterior half, and, generally, by the color of the nacre, though this is not to be relied on. It appears to belong to the genus Symphynota of Mr. Lea.
ALASMODONTA.
13. Alasmodonta complanata, Barnes. Symphynota complanata, Lea. Shell Lake, River St. Croix, Upper Mississippi. Many species of shells found in this lake grow to an extraordinary size. Some of the present collected by Mr. Schoolcraft, measure nineteen inches in circumference.
14. Alasmodontab rugosa, Barnes. St. Croix River, and Lake Vaseux, St. Mary’s River.
15. Alasmodonta marginata, Say. Lake Vaseux, St. Mary’s River: very large.
16. Alasmodonta edentula? Say. Anodon areolatus? Swainson. Lake Vaseux. The specimens of this shell are too old and imperfect to be safely determined.
UNIO.
17. Unio tuberculatus, Barnes. Painted Rock, Upper Mississippi.
18. Unio pustulosus, Lea. Upper Mississippi, Prairie du Chien, to Lake Pepin.