198 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 branchie, 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. 
Secr. [].—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, tom. 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 
