MACLURITES. Lesuour. Journ. des Scienc. Nat. Philad. t. 1. p. 312. pl. 13. fig. 2, 3. MACOMA. Leach. VENUS tenuis, Bl. and similar species, described as "Clothed with an epidermis; striated, compressed, oval; the summits not very prominent; two bifid teeth upon the right valve and a single undivided one upon the left." MACRODITUS. Montf. LENTICULINA, Bl. A genus of microscopic Foraminifera. MACROSPIRA. Guild. A genus composed of HELIX octona, Auct. Macrospira aperta, Guild. MACROSTOMATA. Lam. ([Greek: Makros], _macros_, long; [Greek: stoma], _stoma_, mouth.) A family belonging to the first section of the order Trachelipoda, the shells belonging to which are described as haliotoid or ear shaped, with a very large aperture, destitute of an operculum. This family contains the following genera, which maybe thus distinguished. 1. VELUTINA. Globose, with velvety epidermis. Fig. 337. 2. STOMATIA. Ear-shaped; pearly within; including STOMATELLA. Fig. 335, 336. 3. SIGARETUS. The same, not pearly; including _Cryptostoma_. Fig. 334. 4. CORIOCELLA. The same, thin, transparent. 5. HALIOTIS. The same, not thin, nor transparent; with holes; including _Padollus_. Fig. 338, 339. 6. SCISSURELLA. Heliciform, with a slit near the aperture. Fig. 340. 7. PLEUROTOMARIA. Trochiform, with a slit at the edge of the aperture. Fig. 341. MACTRA. Auct. (_Mactra_, a kneading trough.) _Fam._ Mactracea, Lam. Conchacea, Bl.--_Descr._ 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 cartilage; one long, lateral tooth on each side of the umbo in one valve, received between two in the other; muscular impressions two, lateral; palleal impression with a small sinus.--_Obs._ This genus contains many species of beautiful shells found in various parts of the world, some are common in Britain. Fossil species are not numerous, they occur in the tertiary strata. Fig. 79 to 82. MACTRACEA. Lam. A family of the order Conchifera Dimyaria, Lam. Sect. Tenuipedes. The cartilage placed in a trigonal pit with a small external ligament. The genera may be thus distinguished. 1. LUTRARIA. No lateral teeth, shell gaping. The short species constitute the genus _Ligula_. Fig. 77, 78. 2. MACTRA. Lateral teeth, shell closed. This genus has been divided into Mactra, Mulinia, Schizodesma and Spisula, by Mr. Gray. Fig. 79 to 82. 3. GNATHODON. Teeth serrated, thick, one angular. Fig. 83. 4. CRASSATELLA. Shell thick, lateral teeth. Fig. 84.