ANOD®N  §areolatus. 
Areolated Horse Mussel. 
St rere ETE Boose ae 
GENERIC CHARACTER. 
See Unio, Pl. 1. 
(Anodon. fridina. 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. 
ee UR TE we ee 
Tue 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 
‘Tridina nilotica Sow, “has scarcely any appearance of 
crenations along the hinge line,” but for this, it con- 
sequently would present a simple lameliar 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 4. rwbens Lam. where a lamellar 
plate, though much shorter, is placed beneath the umbones. 
fridina Cailliandi perfectly resembles A. rubens, except in 
being without any vestige of this plate. We have now en- 
tered 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 sufli- 
ciently 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. 
Anodon, P}. 1. 
