19$ MOLLUSCA. 



Corbis, Tellina, Loripes, Lucina, Venus, Capsa, Petricola, 

 Corbula, and Mactra. 



4th Tribe. 



Anterior opening small, and not exposing the mouth or gills. 



In this tribe the mantle is closed in front ; and even when 

 the valves are open, neither mouth nor gills are visible. The 

 anterior opening serves for a passage to the foot, and the 

 posterior openings, in the form of two long tubes, united by 

 a common membrane, serve for the entrance and exit of the 

 water to the mouth and branchiae, and the ejection of the 

 faeces, the dorsal syphon serving the latter purpose. The 

 cuticle of the shell covers also the exposed portion of the 

 cloak, so that, when the animal is removed from the shell, 

 it remains as a loose membrane on the margin of the valves, 

 as was first observed by Reaumur. All the genera prefer 

 concealment, burrowing in sand, mud, or wood, with the 

 head downwards, and the syphons rising to the surface. 

 The following genera belong to this tribe : Mya, Lutraria, 

 Anatina, Glycemeris, Panopea, Pandora, Gastrochena, Bys* 

 somia, Hiatella, Solen, Sanguinolaria, Pholas, Teredo, Xylo- 

 phaga, Clavagella, and Fistulana. 



SECT. II Acephala Tunicata* 



Covering soft or coriaceous. 



The formation of this interesting group of animals was 

 first publicly announced by Lamark in his Histoire Natu- 

 relle des Animaux sans Vertebres, torn. iii. p. 80, (1816.) 

 The labours of Desmarest, Lesueur, and Cuvier, aided by 

 the descriptions of Ellis and Pallis, paved the way for the 

 masterly efforts of Savigny, to whom we owe the most ex- 

 tensive, new, and accurate information yet given concerning 

 the animals of this group. His observations are contained 



