ACEPHALA TESTACEA. 407 



milk-skimmers, but its flesh is not eaten on account of its insi- 

 pidity(l). 

 An oblong species, in which the hinge is granulated throughout 

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

 of Iridina(2); the hind part of its mantle is somewhat closed(3). 



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 



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

 species. 

 Sometimes the anterior tooth is lamiuiform, as in the 



Mya pictorum, L.; Drap., XI, 1, 4. An oblong and thin spe- 

 cies known to every one(4). 

 Lamarck distinguishes the 



Hyria, Lam., 



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

 lares). 



(1) Add, M. anatinus, Chemn., VIII, Ixxxvi, 763; M. fluviatilis. List., clvii, 

 12; M. stagnalis, Schroed., Fluv., I, 1; M.. zellensis, lb., II, 1; if. diibius, 

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

 Test. 



(2) Irid. exotica, Encyc. Method., Test., pi. 204; Add Irid. nilotica, Caillaud, 

 Voy. a M^roe, pi. Ix, f. 11. 



(3) See Deshayes, Mem. de la Soc. d'Hist. Nat. de Paris, 1827, III, p. 1, pi. 1. 



(4) 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 

 estabhshed some new subgenera among them. 



(5) Hyria rugosa, Encyc. Method., ph 247, 2. 



