MYTILACEA. Bl. The fourth family of Lamellibranchiata, Bl. The shells are described as regular, equivalve, frequently with a thick, horny epidermis. A toothless hinge and a linear ligament. This family contains the genera Mytilus and Pinna. MYTILACEA. Lam. A family belonging to the first section of Conchifera Monomyaria, Lam. described as having the ligament partly interior, occupying the greater part of the hinge line, which is straight. The shell is rarely foliaceous. The Mytilaceæ cannot easily be confounded with the Malleaceæ, because the former are generally regular and the latter are irregular, and have a thick internal coating of pearl, beyond which the external coating extends. The genera may be thus distinguished: 1. MYTILUS. Umbones terminating in a point. Fig. 158. 2. DREISSINA. The same, with a septiform plate. Fig. 159. 3. MODIOLA. Anterior margin rounded beyond the umbones. Fig. 160. 4. PINNA. Open at the posterior extremity. Fig. 162. 5. LITHODOMUS. Cylindrical, living in holes. Fig. 161. MYTILUS. Auct. _Fam._ Mytilacea. Lam.--_Descr._ Equivalve, cuneiform, oblique, smooth, with umbones terminal, pointed, and posterior side broad, rounded; hinge linear, with a long, partly internal ligament; muscular impressions two in each valve, that on the posterior side large, irregular; that on the anterior small; palleal impression irregular.--_Obs._ The Linnean genus Mytilus included the Modiolæ, which differ from the Mytili in the rounded anterior side; and the Pinnæ, which are large shells, gaping at the posterior extremity. M. achatinus, fig. 158. NÆARA. Gray. A genus composed of ANATINA longirostrum, Lam. and other similar species. NAIA. Sw. A sub-genus of Castalia, Lam. thus described: "Oval, cardinal teeth beneath the bosses, and deeply sulcated, C. corrugata. _Lam._ En. Méth. 248. f. 8, picta. _Sw._ En. Méth. 248. f. 6." Sw. p. 379. NANINIA. Gray. A genus composed of the planorbicular species of HELIX, with large umbilici, and outer lip thin, included in the sub-genus Helicella, Fer. _Ex._ H. citrina, fig. 280. NASSA. Lam. A genus of small shells united to Buccinum by some authors, but separated by others on account of the little tooth-like projection terminating the columella. N. arcularia, fig. 423.