2 



contractile and extensile, the longer tube (when there are 

 two) being used for respiration and nutrition, and the 

 shorter tube for excretion. The outer edges of the mantle, 

 as well as of the cylinder or tubes, are simple, and not fur- 

 nished with papillae or filaments. The mouth consists of a 

 slit which is placed between the anterior adductor muscle 

 and the base of the foot, and it has two small triangular lips. 

 Foot wedge-shaped, thin, and capable of great extension. 



SHELL composed of two thin, oval or subtriangular valves, 

 which are more or less inequilateral. The valves are of equal 

 size. The outer surface of the shell is protected by a delicate 

 epidermis, and the inside is slightly lined with nacre. The 

 hinge is furnished with cardinal and lateral teeth to enable 

 the valves to lock more closely into each other when the shell 

 is shut. The ligament is external, although it is sometimes 

 seated so far within the hinge as to be scarcely visible on the 

 outside : it is placed at the longer, or posterior, side of the 

 hinge. 



The animals of this family are ovo viviparous, retaining 

 the fry for some time between the mantle and gills. 

 They are tolerably active in their habits, using their foot 

 for crawling like a leech ; and some of them float with 

 the beaks of their shell downwards, or suspend them- 

 selves in that position to the under surface of the water 

 by means of a very fine byssus which they secrete and 

 spin with their foot. In the winter they appear to be 

 torpid, and bury themselves in the mud, like other fresh- 

 water bivalves. During this period they probably cannot 

 procure their food, which consists of animalcula. Speci- 

 mens which I had in confinement soon after Christmas 

 never put out their tubes, and only used their foot to creep 

 under some moss which was in the vessel. This they did 

 as often as I removed them from their place of shelter. 



The Sphariidce closely resemble their marine repre- 

 sentatives, the Kelliadce, which are also ovoviviparous : 

 but the mantle is more open and the ligament external in 

 the present family ; while the ligament is internal in the 



