130 INVERTEBRATA OF MASSACHUSETTS. 



This species, with M. discors, 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 distinc- 

 tive marks between them are particularly pointed out under M. discors. 



The epidermis becomes nearly black by age. 



* 



DISCORS. 



Shell oval, tumid, upper edge somewhat compressed and arching, 

 posterior tip somewhat produced and pointed ; beaks largp, nearly 

 terminal ; surface with about sixteen ribs at the anterior third and 

 very numerous ones at the posterior third. 



FIGURE 84. 

 State Coll., No. 158. Soc. Cab., No. 2344. 



My'tilus discors, LIN ; Syst. Nat., 1159. CHEMN. ; Conch., viii. 191, t. 86, f. 764. 



MONTAGU; Test. Brit., 167. PENNANT; Brit. Zool., iv. 240. MATON and 



RACKETT ; Lin. Trans., viii. 111. t. 3, f. 5. DONOVAN ; Brit. Shells, pi. 25, f. 1. 



DILLWYN; Catal,, 319. TURTON ; Conch. Diet., 112. WOOD; Index, pi. 



12, f. 39. 

 Modiola discrepans, LAM.; An. sans Vert. t vii. 23. DESHAYES ; Encyc. Mdth., 



Vers, ii. 567. 

 Modiola discors, FLEMING ; Brit. Mm., 413. 



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, over- 

 hanging the anterior extremity ; surface as in M. discrepans ; but 

 there are sixteen or more ribs in the anterior compartment, those 

 in the posterior compartment are more crowded, more distinct, 

 the intervening spaces rounded ; and when viewed under the mi- 

 croscope, the whole surface is found to be covered with minute 

 wrinkles of the epidermis crossing the ribs and the spaces be- 

 tween them, and also the middle compartment ; epidermis green- 

 ish-yellow with clouds of olive. Within silvery, margin crenu- 

 lated by the ribs, and with three or four teeth before the beaks. 



