History of British animals - University of Guam Marine Laboratory

Anodon. MOLLUSCA. UNIONISE. 415UNIONID^.Gen. CVI. ANODON.— Shell transverse, rounded at bothextremities, the retral one slightly truncated obliquely ; ligamentexternal, linear, and enlarged at its extremity.424. A. anatinus.— Shell compressed, rising into a wing atthe nearly straight ligament, beaks about one-third of the lengthof 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 littlebent ;beaks about one-fourth of the length of the shell from theextremity.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 runningwater.This species and the former, when in their most distinctive appearances,and nearly of the same size, can be readily distinguished. The A. anatinusis more translucent and fragile, the anterior extremity more compressed ;the hinge-line straighter and the shell rising towards the termination of thehgament, into a more decidedly compressed wing from the :ligament to theextremity, the outline is more oblique, and the anal truncature of the shellmore distinct. In the A. cygneus, there is a convexity in the valves, a roundingof the anterior extremity, and a greater fuhiess of outline. But theseshells differ greatly, according to age and situation, in their forms, as maybe seen in the want of parallelism among the layers of growth ;and the intermediatevarieties are so numerous as to leave little room to doubt thatthe 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 gradationswhich connect the extreme differences in form. Unless the charactersof the animals furnish distinguishing marks, it is worse than useless toload the science with reputed species, depending on variable forms. Accordingto 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.