MODIOLARIA. 



193 



490. 



with dark chestnut-colored shades, folding over the edge. Interior 



of a brilUant silvery lustre ; edge of the two extreme compartments 



crenulated, and very strongly so near the ligament ; a few folds on 



the edge, not corresponding 



to the external ridges, are 



found iust in front of the lis;- 



anient. Length, one inch ; 



heiiiht, thirteen twentieths of 



an inch ; breadth, four tenths 



of an inch. 



Found on Chelsea Beach, 

 and in fishes' maws. Larger 

 specimens arc brought from 

 the Newfoundland Banks. 

 Halifax {WUlis) ; Eastport {Cooper^; fossil at Montreal {Datv- 

 son). I have two specimens which measure one and one half 

 inches in length, and three fourths of an inch in breadth. 



This species, with M. corrus^ata^ is common to the northern 

 coasts of Europe and America, and they are distinguished from all 

 others by the three compartments into which their surface is divided. 

 The distinctive marks between them are particularly pointed out 

 under M. corrugala. The epidermis becomes nearly black by age. 



M. discors (ill. lavigata). 



Modiolaria corrugata. 



Fig. 84. 



Shell oval, tumid, upper edge someAvliat compressed and arching, posterior tip 

 somewhat produced and pointed; beaks large, nearly terminal; surface with 

 about sixteen ribs at the anterior third, and very numerous ones at the posterior 

 third. 



Mocliola discora, Gould, Iiiv. Mass. 1.30, fin;. 84, not of English authors. 

 Myt'das corragatus, Stimpsox, Shells of New England, 12. 



Shell irregularly oval, tumid, heart-shaped when viewed in front, 

 bluntly rounded before ; hinge-margin somewhat ascending and a 

 little compressed ; at the termination of the ligament the 

 margin gradually curves downwards, so that the shell is 

 terminated behind by a lobular, somewhat pointed tip on a 

 level with the base ; basal margin an undulating curve, 

 nearly parallel with the upper margin ; beaks large and 

 prominent, not in contact, overhanging the anterior extremity; sur- 

 face as in 31. discors ; but there are sixteen or more ribs in the 



Fig. 491. 



