104 DIMYARIA. DONAX. 
pit for its reception in the left valve; anterior lateral tooth 
in the right valve obtusely conical ; the posterior tooth in 
the left valve elongated, and attenuated towards the umbo; 
right valve with a flattened fold lying parallel to the liga- 
ment, and divided obliquely near the umbo by a faint groove; 
from the anterior extremity of this fold a depression ex- 
tends beneath the lunule, in front of the anterior lateral 
tooth, with a corresponding elevation ; umbones closely 
approximating; muscular impressions deep; pallial impres- 
sions entire, deeply defined, and destitute of a sinus, liga- 
ment external, situate in a deep, marginal. dorsal sinus. 
Pachyodon crassissima. Plate VIII. fig. 16. 
This is entirely a fossil genus, the species hitherto have been considered and 
described as belonging to the genus Unio, they oceur in the Oolitie group, 
and in the Lias, The difference between Unio and Pachyodon is, that the 
cardinal tooth is frequently obsolete, the anterior lateral tooth thick, simple, 
and destitute of strie, is situated in the right valve, with a hollow for its re- 
ception in the left valve ; the posterior lateral tooth is placed in the left valve, 
and the sulcus for its reception in the right; the two muscular cicatrices are 
very deep, with a small round impression above the anterior one in the left 
valve. They are also destitute of any erosion at the umbo, so common in 
fluviatile bivalves ; and the lines of growth are very strongly marked, 
SUB-DIVISION II. — With one or two lateral teeth, 
Genus XXXIIT. — DONAX, — Linneus. 
Generic Character. — Shell transverse, trigonal, equi- 
valve, inequilateral, outer surface generally covered with a 
thin horny epidermis, anterior side for the most part the 
shorter ; left valve with two more or less distinct cardinal 
teeth; right valve with only one cardinal tooth, which is 
generally cleft at its extremity ; lateral teeth variable, 
either two or one very minute and remote; muscular im- 
pression of the mantle with a large sinus. Ligament exter- 
nal and short. . 
Donazx retusa, Plate IX. fig. 20. 
In some there are two lateral teeth, one of which being placed on each side 
of and near to the primary teeth ; one valve is provided with a linear posterior 
process, remotely situated from the other lateral teeth, and between which 
and the margin of the shell there is a groove for the reception of the teeth of 
the other valve; the anterior lateral tooth can alone be distinguished in each 
valve in other species, together with the linear process ; in other species there 
are two lateral teeth in one valve, the posterior one more remote than the 
anterior, while in the opposite valve there are only slight indications of them ; 
and in some species the lateral teeth are nearly obsolete. The ligament is 
generally short, and in some instances very much so, the greater portion of it 
