Anodon. MOLLUSCA. UNIONISE. 415 



UNIONID^. 



Gen. CVI. ANODON. — Shell transverse, rounded at both 

 extremities, the retral one slightly truncated obliquely ; li- 

 gament external, linear, and enlarged at its extremity. 



424. A. anatinus. — Shell compressed, rising into a wing at 

 the nearly straight ligament, beaks about one-third of the length 

 of the shell from the extremity. 



Musculus latus, List. An. Ang. 146. Conch, t. cliii. — Mytilus anatinus, 

 Linn. Syst. i. 1158. Mont. Test. Brit. 172. Maton and Rackett, Linn. 

 Trans, viii. 110. t. cxi. f. 1. Flem. Edin. Encyc. vii. 99. t. ccv. f. 14. 

 — In ponds and slow running streams. 



Variable in size ; usually with a greenish cuticle worn at the beaks. 



425. A. cygneus. — Shell tumid, line of the ligament a little 

 bent ; beaks about one-fourth of the length of the shell from the 

 extremity. 



M. latus, List. Conch, t. clvi — Mytilus cygneus, Linn. Syst. i. 1158. 

 Mont. Test. Brit. 170. Maton and Rackett, Lin. Trans, viii. 109. 

 t. cxi. f. 2. Flem. Edin. En. vii, 92. t. ccv — In ponds and slow run- 

 ning water. 



This species and the former, when in their most distinctive appearances, 

 and nearly of the same size, can be readily distinguished. The A. anatinus 

 is more translucent and fragile, the anterior extremity more compressed ; 

 the hinge-line straighter and the shell rising towards the termination of the 

 hgament, into a more decidedly compressed wing : from the ligament to the 

 extremity, the outline is more oblique, and the anal truncature of the shell 

 more distinct. In the A. cygneus, there is a convexity in the valves, a round- 

 ing of the anterior extremity, and a greater fuhiess of outline. But these 

 shells differ greatly, according to age and situation, in their forms, as may 

 be seen in the want of parallelism among the layers of growth ; and the in- 

 termediate varieties are so numerous as to leave little room to doubt that 

 the two species should be conjoined. The Mytilus avonensis of Montagu (Test. 

 Brit. 172.) The M. incrassatus and macula of Sheppard (Lin. Trans, xiii. 

 85.) and the Anodon paludosus of Turton (Biv. Brit. 240.) exhibit those gra- 

 dations which connect the extreme differences in form. Unless the charac- 

 ters of the animals furnish distinguishing marks, it is worse than useless to 

 load the science with reputed species, depending on variable forms. Ac- 

 cording to the observations of M. Poiret, as stated by M. Draparnaud (Hist. 

 MolL 134.), the animal of A. anatinus is oviparous, while that of the A. cyg. 

 Tieus is viviparous. 



