MOLLUSCA. 507 



cates the corresponding size of the foot. The foot of the 

 animals of the Iscordia is much larger, and the anterior 

 aperture is large in proportion. In the common cockle (Car- 

 dium), prized by many as an agreeable article of food, the 

 foot occupies a large share of the cavity of the shell. It is 

 bent in the middle, with the point directed forwards. In 

 the genera Cyclas, Tellena, Donax, and Venus, the foot is 

 long and tongue-shaped, and the posterior tubes, in general, 

 considerably produced, and more or less united at the base. 

 In the genus Loripes, the foot is small and cylindrical, and 

 the tubes are short and united. In the Mactra the tubes 

 are likewise short, but the foot is compressed. 



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 visi- 

 ble. 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 ejec- 

 tion of the foeces, the dorsal syphon serving the latter pur- 

 pose. The cuticle of the shell covers also the exposed por- 

 tion of the cloak, so that, when an 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, Byssomia, Hiatella, Sol en, Sanguinolaria, 

 Pholas, Teredo, and Fistularia. 



