11. Anodonta cataracta, Say. Chicago, Lake Michigan. This species, Mr. Lea remarks, has a great geographical extension.
12. Anodonta corpulenta, Nobis. Shell thin and fragile, though less so than others of the genus; much inflated at the umbones, margins somewhat compressed; valves connate over the hinge in perfect specimens; surface dark brown, in old shells; in younger, of a pale dingy green, and without rays, in all I have examined; beaks slightly undulated at the tip. The color within is generally of a livid coppery hue, but sometimes, also, pure white.
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. Alasmodonta 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.