NAIADES. MOLLUSCA. ANODON. 119 



middle ; hinge margin a little curved, forming an angle at both its 

 terminations ; the backward slope from this angle is usually a little 

 curved, and the posterior tip is rather blunt and somewhat trun- 

 cated ; the ridge from the beaks to this tip is very prominent, 

 generally bluntly rounded, but 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. Sur- 

 face rough, with coarse and irregular lines of 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 4 inches, height 2^ 

 inches, breadth l^''- inch. Of another specimen 4| inches, 2Tb- 

 inches, l^o inch ; of another 3 inches, Itct inch, liV inch. 



Inhabits ponds in Essex and Middlesex counties, and is also 

 found in Maine ; whether it occurs southward or not is un- 

 certain. A Pennsylvania shell, which Mr. Lea describes under 

 the name of A. J^ewtonimsis, is so much like some varieties of 

 this shell as to render it probable 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 probably varies more in its form, color, and weight, than any 

 other Anodon. It is undoubtedly the A. im-plicuta of Say, for it ac- 

 cords 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. fluvidtilis ; 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 U'nio 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 margins are similarly curved. Some have a dark tar- 

 colored epidermis ; these are generally very broad in proportion to 

 their height. 



