MYA. DIMY ARIA. 113 
separated in some species, and the ligament placed in a groove or pit extend- 
ing both internally and externally to the beak. 
The Mactre inhabit the seas of almost all climates. Fossil species are 
found in the Secondary and Tertiary formations, especially in the Oolite of 
the former. 
Genus L.— LUTRARIA, — Lamarck. 
Generic Character. — Shell equivalve, inequilateral, thin, 
transversely ovate or oblong ; gaping at both sides ; the pos- 
terior side generally the longer, and always gaping more 
than the other; one valve with two thin laminar teeth, one of 
which is sometimes compound ; the opposite valve with three 
teeth, the central one compound in some instances, and the 
posterior one slender and compressed; two distant lateral 
muscular impressions ; muscular impression of the mantle 
with a large sinus ; ligament internal, situate in a deltoidal 
oblique, internally projecting spoon-shaped pit, with a pro- 
minent margin placed next to the teeth in each valve. 
Lutraria sulcata. Plate XI. fig. 9, Found in the 
Mountain Limestone, Northumberland. 
The Lutrarie are distinguished from the Mactre, by their wanting lateral 
teeth, and the large sinus in the mantle muscular impression, The internal 
ligament removes them from the Anatine. 
The shells of this genus inhabit the sea; and it is quite uncertain if any 
true Lutrarie have been found in a fossil state ; although several species have 
been described as such. 
GRAND-DIVISION IV. — CRASSIPEDES. 
Mantle entirely or partly united before, foot thick, placed posteriorly, 
shell gaping when closed. 
TRIBE I.— MYARIA. 
Ligament internal ; a broad, spoon-shaped tooth in each 
valve, or in one only ; shell gaping at both sides, or at one 
only. 
Genus LI.— MYA. — Zinneus. 
~ Generic Character. — Shell transverse, nearly equivalve, 
gaping at both extremities, but widest at the posterior side ; 
one valve with one large, compressed, dilated, hollow, 
spoon-shaped, perpendicular, vertically projecting tooth; 
the opposite valve destitute of teeth ; the cartilage placed in 
a narrow suture; two lateral, distant, large muscular im- 
pressions, the anterior one narrow, and the posterior almost 
orbicular ; pallial impression with a large sinus; ligament 
H 
