60 MACTRA. <i 
semi-rotund ; impression of the mantle not sinuated. Obs. The 
shells of this genus resemble Amphidesma in general form, but 
are distinguished by the external ligament, the elongated mus- 
cular impression and the want of a sinus in the palleal impression. 
Fig. 104, L. Tigerina. 
LU’NULATE. (Luna, the moon, dim.) Moon-shaped, having 
the form of a crescent. Applied most frequently to muscular 
impressions. Semilunar is sometimes used, perhaps with 
greater accuracy, to express the same shape. 
LUPO'NIA. Gray. A genus composed of species of Cyprma, 
Auct. which are described as having the anterior of the colu- 
mellar lip crossed by several irregular ridges, without any dis- 
tinct marginal ones, internally narrow, flat; the shell pear- 
shaped, smooth or cross-ribbed. Hw. L. Algoensis. Cyprea 
Algoensis, Gray, fig. 447. 
LUTRA’RIA. Lam. (Lutum, mud.) Fam. Mactracea, Lam. 
Descr. Thin, equivalve, inequilateral, transverse, oblong or 
ovate, gaping at both extremities; hinge with one double and 
sometimes one single cardinal tooth in each valve, and a trian- 
gular oblique pit with a prominent margin, containing the liga- 
ment; muscular impressions distant; palleal impression having 
a large sinus. Obs. This genus differs from Mactra, in the en- 
tire absence or slightness of lateral teeth. Fig. 77, L. Papyra- 
cea; Ligula, Leach. Fig. 78, L. Solenoides. 
LUTRI'COLA. Bl. Lurraria, Lam. Fig. 77, 78. 
LY’COPHRIS.* Montf. A microscopic fossil described as resem- 
bling Nummuites, but having a granulated surface. 
LYMNA’‘DIA. Sw. Unio Gigas, Lea. 
LYMNEA, Limvnza. 
LYMNEA’NA. Lam. A family belonging to the first section of 
the order Trachelipoda, Lam. containing the genera Planorbis, 
Physa and Limnea. Fig. 308 to 312. 
MACO’MA. Leach. Venus tenuis, Bl. and similar species. 
MACRODITUS. Montf. Lenticutina, Bl. A genus of micro- 
scopic Foraminifera. 
MACROSTOM ATA. Lam. (Maxpos, macros, long; sroua, stoma, 
mouth.) A family belonging to the first section of the order 
Trachelipoda, containing the genera Sigaretus, Stomatella, Sto- 
matia, Haliotis. ‘To these may be added several genera enume- 
rated in explanation of figures 334 to 341. 
MAC’TRA. Lam. (Mactra,a kneading trough.) Fam. Mactra- 
cea, Lam. Conchacea, Bl. Deser. Usually thin, equivalve, sub- 
equilateral, sub-trigonal, slightly gaping at the extremities; hinge 
with one cardinal tooth, divided into two parts, diverging from 
the umbo, with sometimes a very small laminar tooth close to its 
side; a deep triangular pit near the centre, containing the car- 
tilage; one long, laminar, lateral tooth on each side of the um- 
bo in one valve, received between two in the other; muscular 
i, 
