MONOPLEUROBRANCHIATA. 191 



MONOCEROS. (Moj'oe, monos, single; KspaQcei'as, horn.) Fam. 

 Purpurifera, Lam. — Bescr. Ovate, thick, covered with a brown 

 epidermis ; spire short, consisting of few whorls ; aperture emar- 

 ginated anteriorly ■ columella rather flat ; outer lip thick, with 

 a prominent tooth near the extremity. — Obs. This genus resem- 

 bles Purpura, in every respect, except in having the tooth 

 from which the name is derived. A catalogue of 1 6 species by 

 Mr. Sowerby, sen. is published with figures of 14, in parts 58 to 

 67 of the Conchological Illustrations by the author. The species 

 belong to the South American coasts of the Pacific Ocean. 



MONOCONDYLm. D'Orb. A sub-genus of Uniones, described 

 as equivalve, inequilateral, sub-rotund or angulated ; hinge con- 

 sisting of a large, obtuse, round cardinal tooth in each valve, with 

 no lateral teeth. Monocondylse (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 Paracephalo- 

 phora, Bl. divided into the orders Pulmobranchiata, Chismo- 

 branchiata, Monopleurobranchiata, in the first section ; and 

 Aporobranchiata, Polybranchiata, Cyclobranchiata, Inferobran- 

 chiata, and Nucleobranchiata, in the second. 



MONOMYARIA. Lam. (Movoe, monos, single; /xvoy, 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, con- 

 taining the families Pectinides, Ostracea; third section, con- 

 taining 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 opercu- 

 liform mantle, in which there is sometimes enveloped either a flat 



