FAMILY 12. NAIADES. 117 



The shell of Unio may be described as being transverse, generally 

 thick and solid, equivalve, inequilateral, and covered with an olivaceous 

 epidermis. The interior of the valves is lined with an iridescent nacre, 

 which is sometimes white, sometimes pink ; the colour, however, though 

 constant in some species, often varies in different individuals of the same. 

 The hinge is composed of two solid lateral teeth : one, which has almost 

 the appearance of a cardinal tooth*, is short, simple, sometimes bipartite, 

 and somewhat striated ; the other is produced anteriorly and rather long. 

 The posterior muscular impression is always compound, and the ligament 

 is external. 



Examples. 



PI. LXXXV1I. Fig. 1. 



Unio ovalis, Fleming. Wood, Gen. Conch., pi. 19. f. 5. Draparnaud, 



Moll. terr. et fluv. de France, pi. 11. f. 1 to 4. 

 Mya ovalis, Montague. 

 Mya ovata, Maton. 

 Unio pictorum, var., Lamarck. 

 Var. Unio Michaudii, Desmoulin. 



PI. LXXXVII. Fig. 2. 



Unio ambiguus, Sowerby, Genera of Shells, No. 16. De Blainville. 



Manuel de Malacologie, pi. 67. f. 4. 

 Castalia ambigua, Lamarck. Gray. D'Orbigny. 

 Mya ambigua, Wood. 



PI. LXXXVIII. Fig. 3. 



Unio clavus, Lamarck, Anim. sans vert., new edit., vol. vi. p. 537. 

 Unio modioliformis, Say. 

 Unio scalenius, Rafinesque. 



* This is actually described by Lamarck as a cardinal tooth : it cannot be denied but that it 

 is literally cardinal, as forming part of the hinge (cardo), but it is certainly not analogous to 

 the central tooth or teeth, technically distinguished by the term cardinal in describing the 

 himre of bivalve shells. 



