FAMILY NAIADES. 



223 



3. Unio margaritifer. Pearl-bearing Unio. 



Shell ; oblong or elongately kidney-shaped, very inequilateral, 

 rounded at both ends, anterior end a little gaping in the region 

 of the foot, valves moderately convex, often compressly pinched 

 about the middle, covered with a black densely fibrous epider- 

 mis, umboes mostly very much eroded, ligament long and 

 strong, hinge composed of a blunt crested anterior tooth in the 

 right valve, interlocking somewhat rudely with a double, simi- 

 larly crested tooth in the left valve, spreading at the base in a 

 thickened deposit towards the front adductor muscle, posterior 

 tooth represented in each valve by an obtuse marginal ridge. 



Mya inargaritifera, Linnaeus (1758), Syst. Nat. ed. 10. p. 671. 



Unio margaritiferus and crassus, Pliilippson (1788), Nov. Test. Gen. p. 16. 



Unio rugosa, Poiret (1801), Coq. de I'Aisne, p. 105. 



Margaritana fiuviatilis, Schumacher (1817), JSss. Syst. Test. p. 124. 



Unio elongata and sinuata, Lamarck (1819), Anini. sans vert. vol. vi. part 

 i. p. 70. 



Unio elongatus and ater, Nilsson (1822), Moll. Suec. p. 106, 107. 

 Alasmodonta arcuata, Barnes (1823), Sillim. Journ. vol. vi. p. 277. pi. xi. 

 f. 20. 



Unio crassissima, Ferussac (1827), Desmoul. Moll. Girond. p. 42. 

 Alasmodon margaritiferum, Fleming (1828), Brit. Anim. p. 417. 



