2 SPH^RIID^. 



extensile, — the longer tube (when there are two) being used 

 for respiration and nutrition, and the shorter tube for excre- 

 tion. The outer edges of the mantle, as well as of the cylinder 

 or tubes, are simple, and not furnished with papilla or fila- 

 ments. 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 Hps. Foot wedge-shaped, thin, and ca- 

 pable 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 siu^face of the shell is protected by a delicate 

 epidermis, and the inside is sHghtly 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 ovoviviparous^ 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- 

 Avater bivalves. During this period they probably cannot 

 procure their food, w hicli 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 Sphm'ndcB closely resemble their marine repre- 

 sentatives, the KeIHad(Bj which are also ovoviviparous : 

 ])ut the mantle is more open and the ligament external in 

 the present family; while the ligament is internal in the 





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