104 CYCLADID^. 



SO as to form a conspicuous obtuse angle, causing this end to ap- 

 pear higher, and giving the wliole sliell a somewhat rhombiform 



appearance ; basal margin regularly rounded ; 

 ^'"' ^^' valves very tumid, especially in mature shells ; 



surface shining, inconspicuously wrinkled by the 

 lines of growth, with very indistinct, radiating 

 lines ; color of the young very light-yellowish ; of 

 the adult light-greenish horn-color, with a mar- 

 ginal border of lilac or yellow ; cardinal teeth 

 '^■^E^ar"ea""" suiall, diverging; marginal teeth strong, white ; 



interior tinged with lilac. Length, nine twen- 

 tieths of an inch ; height, two fifths of an inch ; breadth, four fif- 

 teenths of an inch. 



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

 more usually imbedded among submerged turf and roots of water- 

 plants and shrubs. Truro, Nova Scotia {McCulloch}; nearly all 

 lakes. Nova Scotia (^ Will is). 



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, with- 

 out any angle or any widening behind. Our shell is, on the whole, 

 more delicate. 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 parts is much greater than in the adult. They have also 

 a light honey-yellow color and great transparency. They would 

 scarcely be recognized as the same species except by being found 

 in company, and also by being actually found within the adult shell. 



Sphaerium rhomboideum. 



Fig. 55. 



Shell rhombic-orbicular, tumid, beaks not prominent, sub-equipartito ; color 

 olivaceous, margined with yellowish ; surface elegantly marked with fine concen- 

 tric ridges. 



Ci/clas rhomhoiden, Say, Journ. Acad Nat. Sc ii. 380 (1822). — Prime, Ann. N. Y. Lvc. 



vi. 06, pi. 1, fiff. 4(/, /* (185.-?) ; Troc. Bost. Soc. iv. 272 (185->). 

 Sphfprlum r/wwhairlmm, Adams, Gen. ii. 450 (1858). — Prime, Proc. Ac. Nat. Sc. xi. 297 



(1860), xii. 406. 

 Sphcerlum ehfjanx, H. and A. Ada^ms, Gen. ii. 450 (1858). 



