In the sketch already given of this group, we have placed the genus Anodon between Iridina and Alasmodon. The Iridina nilotica Sow, "has scarcely any appearance of crenations along the hinge line," but for this, it consequently would present a simple lamellar ridge, extending nearly the length of the shell; such, in short, as is seen in Dipsus plicatus (Leach), whose tooth may be so described. By this shell we are led to A. rubens Lam. where a lamellar plate, though much shorter, is placed beneath the umbones. Iridina Cailliandi perfectly resembles A. rubens, except in being without any vestige of this plate. We have now entered among the typical species, where every form occurs between a round and an ensiform shape. Leaving these, Nature proceeds to fashion an indistinct developement of Alasmodon in the species before us, where that part of the hinge plate, immediately beneath the umbones, is somewhat raised, and undulated. In A. rugosus these compressed undulations assume more the form of tubercles; and finally, in the Alasmodon marginatus of Say, the teeth are sufficiently developed to place that shell in a distinct group.
Our esteemed and accomplished friend, Mrs. Corrie of Birmingham, favoured us with two examples of what appears to be a variety of that which we have figured. The substance of both is opake; the inside white, tinged with buff, and with scarcely any pearly lustre. Inhabits North America, but we know not any precise locality.
Pl. 19.
MITRA. Pl. 2.
1. strigata. 2. bicolor. 3. carinata.
MITRA strigata.
Specific Character.
Shell very smooth, chestnut with paler longitudinal stripes, which are white at their commencement; aperture white, shorter than the spire; pillar 4-plaited.