204 MOTHEB OP PEAEL. 



in each valve. The Monomyaria are thus divided: First sec- 

 tion, containing the families Tridacnacea, MytHacea, MaUeacea; 

 second section, containing the families Pectiaides, Ostracea j 

 third section, containing the families Eudistes, Brachiopoda. 



MONOPLEUEOBEANCHIATA. 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 pateUiform shells, is divided into the following families : 

 Fam. 1. Subaplysiacea ; 2. Aplysiacea ;. 3. PateHoidea ;, 4. 

 Acera. 



MOISTOPTYGMA. Lea. A genus of small shells resembling Tor- 

 nateUa, but having a strong, oblique fold in the centre of the 

 columellar lip. M. elegans. PL xv. fig. 344. 



MONOTHALAMIA. {Movoq, monos, single ; daXafiog, thalamos, 

 chamber.) The second division of Cephalopoda, Lam. contain- 

 ing only one genus, namely Argonauta. 



MOISTOTHYEA. A term used by. Aristotle to designate spiral 

 imivalves. 



MONOTIGtMA. Grray. A genus founded on the species repre- 

 sented. It is a turrited sheR, but we are unacquainted with 

 the characters of the genus. PL xvi. fig. 371. 



MONTACUTA. Turton, 1822. (See Kelltad^.) M. bidentata. 

 PL xxvii. fig. 570. 



MOPALIA. Gray. Chitojt, Hindsii, &c. 



MOEIO. Montf. 1810. Cassidaeia, Lamarck, 1812. (The 

 former should be used.) C. Echinophora, fig. 407. 



MOTHEE OF PEAEL. This beautiful substance, which is so 

 much resorted to for ornamental purposes, constitutes the 

 thickened coatiag of the internal surface of the shell named by 

 scientific collectors, Meleagrina Margaritifera, commonly called 

 the Pearl Oyster, a young specimen of which is figured (164) in 



