ANODON. 181 



and the posterior tip is rather l)hint and somewhat trnncated ; the 

 ridge from the beaks to this tip is very prominent, generally bluntly 

 rounded, Init sometimes quite abrupt ; the space above it is rough, 

 but is little compressed, except in young specimens ; three or four 

 coarse lines often run along this space in the direction of the ridge ; 

 basal margin very gently curved in young specimens, nearly parallel 

 with the hinge-margin in the middle-aged, and deeply contracted or 

 arched, in old shells. Surface rough, with coarse and irregular lines 

 uf growth ; epidermis yellowish-olive, darker above and behind, and 

 with dusky-brown zones ; young shells are of a delicate grass-green, 

 slightly rayed. Interior silvery till after the middle age, when it 

 becomes of a delicate flesh-color or salmon-color. Length, four 

 inches ; height, two and one fourth inches ; breadth, one and seven 

 twentieth inches. Of another specimen, four and one half inches ; 

 two and four tenth inches ; one and nine tenth inches. Of an- 

 other, three inches ; one and six tenth inches ; one and one tenth 

 inches. 



Inhabits ponds in Essex and Middlesex counties, and is also 

 found in Maine ; whether it occurs southward or not is uncertain. 

 A Pennsylvania shell, which Mr. Lea describes under the name of 

 A. Ncirf.onicnsis, is so nmch like some varieties of this shell as to 

 render it probaljle that they are the same, and that it is found 

 throughout a wide southern range. 



The above is the description of a characteristic specimen of a 

 shell which probaldy varies more in its form, color, and weight 

 than any other Anodun. It is undoubtedly the A. implicata of Hay, 

 for it accords well with his description, and was received from a 

 region where no other species is found. In their younger stages it 

 is difficult to distinguish them from A. fiuviatiUs ; but the great 

 thickening near the margin which the adult undergoes, and its 

 light-yellowish epidermis, render them entirely dissimilar. I have 

 specimens in which portions of the valves are three tenths of an 

 inch thick. At the middle age, some specimens so much resemble 

 very old ones of Unio radiatus, that it is impossible to name them 

 without examining the hinge. In the young, the beaks are delicately 

 undulated, the hinge-margin is compressed and connate, and the 

 angle at its posterior termination is very decided. Some specimens, 

 of a middle size, lose all their angles, and the upper and lower mar- 

 gins are similarly curved. Some have a dark, tar-colored epider- 

 mis ; these are generally very broad in proportion to their height. 



