242 MALACOZOA. TROPIOPODA. LAMILUBRANCHIATA. 



height, drop the shell on a rock, and pick out the meat, when 

 the shell is fractured by the fall." 



Mytilus anatinus. Linn. Syst. Nat. i. 1158. Mytilus anatinus. 

 Perm. Zool. iv. 113. PI. 68; Ed. n. iv. 239. PL 71. Mytilus 

 anatinus. Mont. Test. Brit. 171. Anodonta anatiua. Lamk. 

 Anodon anatinus. Turt. Biv. 240. Anodon anatinus. Flem. Brit. 

 Anim. 415. 



GENUS 2. ALASMODON. PEARL-MUSSEL. 



Shell regular, equivalve, ovate or oblong, very ine- 

 quilateral, compressed, concentrically rugose, with a 

 dense epidermis. Umbones small, incurved, carious. 

 Hinge with an irregular prominent crenate tooth in the 

 right valve, entering between two similar teeth in the 

 left. Ligament external, linear, elongated, strong. An- 

 terior margin rounded ; posterior subangulate. 



The species reside in mud or gravel, in rivers. 



1 . Aldsmodon margaritiferus. OHong Pearl-Mussel. 



Shell transversely ovato-oblong, rather compressed, thick, 

 finely striate longitudinally, transversely rugose, with the pos- 

 terior margin sloped ; the umbones carious ; the teeth strong, 

 conical, compressed, somewhat lobed ; the epidermis blackish- 

 brown ; the inside bluish-white, and pearly. Length six 

 inches, height three. 



It varies much in size, form, and colour. Young indivi- 

 duals are ovato-oblong, or ovate, with the epidermis olivaceous. 

 Middle-aged individuals are of nearly the same form, but 

 longer in proportion to their height, with the valves thick, the 

 umbones decorticated or corroded, the epidermis dark-brown. 

 In old individuals, some of which are from five to six inches 

 in length, the lower margin is widely sinuate, so as to give the 

 shell a curved appearance. The valves are very thick in old 

 shells, their interior pearly, bluish, or sometimes tinged with 

 red, the epidermis nearly black. Pearls of various sizes, 

 forms, and colours, are found in this species : spherical, he- 

 mispherical, binate, roundish, oblong ; from a twelfth or less 

 to half an inch in diameter ; white, bluish, pink, or dusky. 



Common in the Dee, the Don, the Ythan, the Ugie, and the 

 Doveran, in muddy and gravelly places. 



Mya margaritifera. Muller, Verm. Terrest. et Aquat. ii. 210. 

 Mya margaritifera. Linn. Syst. Nat. i. Mya margaritifera. Mont. 



