MARINE MOLLUSCA OF THE UNITED STATES. 189 



2. C. PABA, Fabricius. Fig. 516. 



(Mytilus.) Fauna Groenl. 1780. 

 Modiola pectinula, Gould, Invert. Mass., 1st edit. 127, t. 85. 



Shell obovate, ventricose, with about forty equal radiating 

 ribs ; beaks prominent, projecting as far as the anterior margin ; 

 margin crenulated within ; epidermis brownish-yellow. 



Length 17.5, breadth 12.5 mill. 



Arctic Seas to Greenland. 



b. Sides of the shell with radiating lines, middle portion smooth. 

 S. G. MODIOLARIA, Gra}'. 



3. C. NIGRA, King. Figs. 517, 518, 519. 



Ann. Nat. Hist., xviii. 239. 



Mytilus discrepans (?) Mont., Test. Brit. Suppl., 65, t; 26, f. 4. 

 Modiola nexa, Gould, Invert. Mass., 1st edit. 128, f. 86 



Shell ovate ; beaks prominent, and placed considerably behind 

 the anterior extremity; epidermis rusty-brown. 



Length 2.5, breadth 1.5 inches. 



Massachusetts, northwards. 



The young shell (Fig. 518), represents M. nexa, Gould. 



4. C. DISCORS, Linnaeus. Figs. 520, 521. 



(Mytilus.) Syst. Nat., edit. xii. 1159. 1767. 

 Mytilus discrepans, Mont., Test. Brit., 169. 1803. 

 Modiola Icevigata, Gray, App. Parry's 2d Voyage, 245. 1824. 



Shell suboval, broadest behind; beaks nearly terminal ; hinder 

 extremity somewhat lobed ; anterior ribs about eight ; posterior 

 ones numerous; greenish-yellow, with clouds of olive, becoming 

 nearly bla*ck in old specimens. 



Length 37, breadth 20 mill. 



Massachusetts, northwards. (Eur.) 



The smaller figure represents discors (not fully grown), and the 

 large figure is from an adult specimen (Gray*s Isevigata). 



5. C. CORRUGATA, Stimpson. Fig. 522. 



(Mytilus.) Shells N. E., 12. 1851. 



Modiola discors, Gould, Invert. Mass., 130, f. 84 (not of English authors.) 

 Shell oval, tumid; upper edge somewhat compressed and arch- 

 ing; 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 ; three or 

 four teeth before the beaks; epidermis greenish-yellow. 



Massachusetts, northwards. 



