MONOCONDYLÆA. D'Orb. A sub-genus of Uniones, described as equivalve, inequilateral, sub-rotund or angulated; hinge consisting of a large, obtuse, round cardinal tooth in each valve, with no lateral teeth. Monocondylæ (Unio) Paraguayana, D'Orb. fig. 149. MONODONTA. Lam. ODONTIS, Sow. A genus separated from Trochus, Auct. on account of the tooth or notch with which the columella abruptly terminates. M. labeo, fig. 366. MONOICA. Bl. The second sub-class of the class Paracephalophora, Bl. divided into the orders Pulmobranchiata, Chismobranchiata, Monopleurobranchiata, in the first section; and Aporobranchiata, Polybranchiata, Cyclobranchiata, Inferobranchiata, and Nucleobranchiata, in the second. MONOMYARIA. Lam. ([Greek: Monos], _monos_, single; [Greek: muon], _myon_, muscle.) The second order of Conchifera, consisting of those bivalve shells which have but _one_ principal muscular impression in each valve. The Monomyaria are thus divided: First section, containing the families Tridacnacea, Mytilacea, Malleacea; second section, containing the families Pectinides, Ostracea; third section, containing the families Rudistes, Brachiopoda. MONOPLEUROBRANCHIATA. Bl. The second order of the first section of Paracephalophora Monoica. Bl. The animals are described as having the lungs branched, situated at the right side of the body and covered more or less completely by the operculiform mantle, in which there is sometimes enveloped either a flat or a more or less involute shell, with a large entire aperture. They have either rudimentary or auricular tentacula, or none. This order, which includes mollusca with haliotoid or patelliform shells, is divided into the following families: _Fam._ 1. Subaplysiacea; 2. Aplysiacea; 3. Patelloidea; 4. Acera. MONOPTYGMA. Lea. A genus of small shells resembling Tornatella, but having a strong, oblique fold in the centre of the columellar lip. M. Elegans, fig. 344. MONOTHALAMIA. ([Greek: Monos], _monos_, single; [Greek: thalamos], _thalamos_, chamber.) The second division of Cephalopoda, Lam. containing only one genus, namely Argonauta. MONOTHYRA. A term used by Aristotle to designate spiral univalves. MONOTIGMA. Gray. A genus founded on the species represented fig. 371. It is a turrited shell, but we are unacquainted with the characters of the genus. MORIO. Montf. CASSIDARIA, Auct. C. Echinophora, fig. 407.