110 DIMYARIA. AMPHIDESMA. 
subequilateral, transverse, generally gibbose and close; 
each valve usually furnished with a single conical, recurved, 
ascending, pointed tooth, at the side of which is a small 
concave depression, — very deep in some species, which 
serves either for the reception of the ligament, or the tooth 
of the opposite valve ; two distant, lateral, somewhat irregu- 
Jar muscular impressions in each valve; pallial impression 
posteriorly angulated, with a very small sinus ; ligament 
internal, fixed to the tooth of the lesser valve, and inserted 
in the depression by the side of the tooth, in the larger 
valve. 
Corbula revoluta. Plate X. fig. 10. 
The Corbule inhabit the ocean, and are pretty numerous. They are fre- 
quently found in a fossil state in the English Crag, Greensand and London 
Clay, and contemporaneous formations. 
The shells of this genus differ from those of Mya, in having a sinus in the 
pallial impression, and in their prominent ligamentiferous tooth in each valve, 
while the Myz have only one. 
TRIBE IV.— MACTRACEA. 
Shells equivalve, often gaping at the lateral extremities ; 
ligament interior, or partly external ; animal with the foot 
small and compressed. 
SUBDIVISION I. — Ligament seen externally or double. 
Genus XLV. AMPHIDESMA.— Lamarck. 
Generic Character. — Shell equivalve transverse, slightly 
inequilateral, somewhat ovate or orbicular ; some species gap- 
ing at the sides ; each valve provided with one, or in some 
instances two small, slender, cardinal teeth ; and two dis- 
tinct elongated, lateral teeth, situate near the hinge in one 
valve, and are nearly obsolete in the other; pallial impres- 
sion with a very large sinus ; ligament double, its external 
portion slender, and rather short, and the internal cartilage 
generally longer and larger, adherent in both valves to an 
elongated groove or pit, which varies in length in different 
species, and takes its rise immediately within the umbo, and 
is prolonged within tie anterior lateral tooth. 
Amphidesma decaratum. Plate IX. fig. 8. 
The larger species of this genus are provided with a flexure in the anterior 
margin of both valves, as in the Telline, 
The inner portion of the ligament being at a distance from the cartilage, 
distinguishes the shells of this genus from all others, because in most uni- 
valves the cartilage and ligament are united in one mass, or situated close to 
