ACEPHALA TESTACEA. 95 



anus obtuse and almost rectilinear ; the hinge of the thin and mode- 

 rately convex shell has no appearance of a tooth whatever, being 

 merely furnished with a ligament which extends along the whole of 

 its length. The animal, — LimNjEa, Poli, has no byssus ; its foot, 

 which is very large, compressed and quadrangular, enables it to 

 crawl upon the sand or ooze. The posterior extremity of its mantle 

 is provided with numerous small tentacula. The Anodontes inhabit 

 fresh water. 



Several species are found in France, one of which — Mytilus 

 cygneus, L., Chemn., VIII, Ixxxv, 762, is common in ponds, &c., 

 with oozy bottoms. Its light and thin shells are used for milk- 

 skimmers, but its flesh is not eaten on account of its insipidity*. 



An oblong species, in which the hinge is granulated throughout 

 its whole length, is distinguished by M. de Lamarck under the name 

 of iRiDiNAf ; the hind part of its mantle is somewhat closed |. 



Dr. Leach distinguishes another by that of Dipsada, where the 

 angles are more decided, and in which there is a vestige of a tooth on 

 the hinge. 



Unio, Brug. 



These Mollusca resemble the Anodontes both in their animal and 

 shell, with the exception of their hinge, which is more complex. 

 There is a short cavity in the anterior part of the right valve, which 

 receives a short plate or tooth from the left one, and behind it is a 

 long plate which is inserted between two others on the opposite side. 

 They also inhabit fresh water, preferring running streams. 



Sometimes the anterior tooth is more or less stout and unequal, 

 as in 



My a margaritifera, L. ; Drap., X, 17, 19. A large thick spe- 

 cies, the nacre of which is so beautiful that it is employed as 

 pearls. Found in France ; as is the 



Vino littoralis. Lam., Drap., X, 20. A smaller and square 

 species. 

 Sometimes the anterior tooth is laminiform, as in the 



My a pictorum, h. ; Drap., XI, 1, 4. An oblong and thin 

 species known to every one §. 

 Lamarck distinguishes the 



Hyria, Lam., 



In which the angles are so decided that the shell is nearly trian- 

 gular ||. 



* Add, M. anatinus, Chemn., VIII, Ixxxvi, 763; — M.fluviatilis, l.ist., clvii, 12 ; 

 — M. stagnalis, Schroed, Fluv., I, 1 ; — M. zellensis, lb., II, 1 ; — M. dubius, Adans., 

 XVII, 21 ; and the pi. 201, 202, 203, and 205, of the Encyc. Method., Test. 



f Irid. exotica, Encyc. Method., Test., pi. 204 ; — Add Irid. nilotica, Caillaud, 

 Voy. k M^ro^, pi. Lx, f. 11. 



+ See Deshayes, M6m. de la Soc. d'Hist. Nat. de Paris, 1827, III, p. 1, pi. 1. 



§ Numerous species, remarkable for size or form, inhabit the rivers and lakes of 

 the United States. Messrs. Say and Barnes, who have described them, have estab- 

 lished some new subgenera among them. 



II Hyria rugosa, Encyc. Method., pi. 247, 2. 



