Anodon. 
MOLLU SC A. UNIONIDiE. 
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 extremit}\ 
424. A. andtinus. — ‘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 Racked 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, Lirm. Syst. i. 1158. 
Mont. Test. Brit. 170. Maton and Racked 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 fulness 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 amnensis of Montagu (Test. 
Brit. 172 .) The M. inorassatus 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^ 
neus is viviparous. 
