Zoological Illustrations Series II/Plate 18
Anodon, Pl. 1
A. areolatus.
ANODON areolatus.
Areolated Horse Mussel.
Generic Character.
See Unio, Pl. 1.
(Anodon. Iridina. Dipsus. Auct.)
Specific Character.
Shell transversely oval, not winged, umbones small, ventricose, not touching; with a short undulated projection of the hinge margin in one valve, and a corresponding concavity in the other; both placed beneath the umbones.
The principal character of Anodon, rests on the absence of teeth; the A. anatinus, common to our rivers and ponds, being taken as the type of the group. But, as Nature steps progressively, in her departure from one form, and in her advance to another, so among the Anodons we find several shells, which do not strictly exhibit the typical character; and these we shall briefly notice.
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.