192 MOURETIA. 



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. Sub- 

 aplysiacea ; 2. Aplysiacea ; 3. Patelloidea; 4. Acera. 

 MONOPTYGMA. Lea. A genus of small shells resembling Torna- 

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

 columellar lip. M. Elegans, fig. 344. 

 MONOTHALAMIA. (Movoq, monos, single; OaXafiog, thalamos f 

 chamber.) The second division of Cephalopoda, Lam. containing 

 only one genus, namely Argonauta. 

 MONOTHYRA. A term used by Aristotle to designate spiral uni- 

 valves. 

 MONOTIGMA. Gray. A genus founded on the species represented 

 fig. 3/1. It is a turrited shell, but we are unacquainted with the 

 characters of the genus. 

 MORIO. Montf. Cassidaria, Auct. C. Echinophora, fig. 407. 

 MOTHER OF PEARL. This beautiful substance, which is so 

 much resorted to for ornamental purposes, constitutes the 

 thickened coating of the internal surface of the shell named 

 by scientific collectors, Meleagrina Margaritifera, commonly cal- 

 led the Pearl Oyster, a young specimen of whieh is figured (164) 

 in our plates. The reason why this substance is called mother- 

 of-pearl is that the true pearls are produced from its surface. 

 They arise principally from accident or disease, and are some- 

 times artificially produced by pricking through the outside of 

 the shell while the animal is living. The animal is allowed 

 to live until it has formed a pearl over the wounded part. 

 MOULINSIA. Grateloup. Pupina, Vignard. A genus of small 

 land ^shells with enamelled surface and spiral operculum. See 

 Pupina. 

 MOURETIA. Gray. " Gadin," Adanson. A genus of patelli- 

 form shells, described as differing from Siphonaria (the original 

 Mouretia of Adanson) in the situation of the siphon, which in 

 Mouretia is close to the place where the muscular impression 



