74 INVERTEBRATA OF MASSACHUSETTS. 



Shell rounded-oval, broadest behind, thin, fragile, pellucid, 

 somewhat inflated ; beaks nearly central, moderately elevated, 

 inclining inwards; hinge margin nearly straight, passing by a 

 regular curve into the rounded anterior extremity, but curving 

 suddenly behind, so as to form a conspicuous obtuse angle, caus- 

 ing this side to appear broader, and giving the whole shell a some- 

 what rhombiform appearance ; basal margin regularly rounded ; 

 valves very tumid, especially in mature shells ; surface shining, 

 inconspicuously wrinkled by the lines of growth, with very indis- 

 tinct radiating lines ; color of the young very light yellowish ; of 

 the adult light-greenish horn-color, with a marginal border of lilac 

 or yellow ; cardinal teeth small, diverging ; lateral teeth strong ; 

 white ; interior tinged with lilac. Length / inch, height f inch, 

 breadth T \ inch. 



Found everywhere in fresh water brooks and ditches, in mud, 

 or more usually inbedded among submerged turf, and roots of 

 water-plants and shrubs. 



The animal is of a light, delicate pink color, and draws itself 

 about rapidly. 



The general resemblance of this shell to the C. cornea of Europe is 

 very close. Its size, color, delicacy, and tumid form are the same. 

 But that species has the beaks much less elevated, is broader from 

 side to side, and the two ends are almost precisely alike, without any 

 angle, or any widening behind. Our shell is, on the whole, more deli- 

 cate. It is unquestionably the variety noticed by Lamarck as coming 

 from America. The young and old differ both in shape and color. 

 The young are less tumid and longer, and the disparity of the sides is 

 much greater, than in the adult. They have also a light honey-yellow 

 color and great transparency. They would scarcely be recognised as 

 the same species except by being found in company, and also by 

 being actually found within the adult shell. 



CYCLAS E'LEGANS. 



Shell rhombic-orbicular, tumid, beaks not prominent, sub-equi- 

 lateral; color olivaceous, margined with yellowish ; surface ele- 

 gantly marked with Jinc concentric ridges. 



FIGURE 55. 



