Fam. 4.] NATUKAL HISTORY SOCIETY. 89 



umbones rather prominent, usually eroded quite a distance from 

 the anterior end ; one section of the fine radiations covers the 

 whole upper and posterior part of the shell ; the second section, 

 at the anterior end, is small in extent but distinctly radiated ; 

 margin not crenulated. Epidermis rusty brown, glossy ; interior 

 livid, with pearly or silvery lustre. L. 50; H. 28; W. 15; U. t\). 

 — Connecticut and north. Deep water. 



2. Modiolaria lateralis. Say. Shell oval, widest near the mid- 

 dle ; gonial ridge broad, rounded, and forming a rounded lobe 

 at the gonium ; on and above this ridge, the surface is covered 

 with numerous, fine, radiating lines, ; in front of the umbo, there 

 are about 8 small, rib like radiating lines. Epidermis olive, 

 with dark, chestnut colored shades ; interior brilliantly silvery. 

 L. 30 ; H. 18 ; W. 12 ; U. A.— Maine and south. 



3. Modiolaria corrugata, Stim. Shell irregularly oval, tumid, 

 heart-shaped when viewed in front; somewhat lobed at the 

 gonium, as in the preceeding; about 16 ribs in the front section, 

 and many over the back ; margin crenulated. Epidermis greenish 

 yellow ; within, silvery. L. 11 ; H. 7 ; W. 4 ; U.ro. — Long Island 

 and north. Deep water. 



4, Crenella, Br. 



Shell oval or rhomboidal, pearly, with radiating ribs ; umbones 

 straight ; ligament small ; a crenulated cardinal tooth in each 

 valve. 



Animal with a cylindrical foot having a disk like end issuing 

 out of a sheath ; byssus a single thread. 



1. Crenella glandula, Tot. Shell rounded oval; umbones slightly 

 projecting and (if the shell is held in the mussel position), near 

 the anterior end ; surface covered with minute lines of growth, 

 crossed by many, small, rounded, radiating ribs, nearly equal in 

 size, over the whole surface of the shell, the number increasing 

 as the spaces widen. Epidermis thin, brownish yellow ; within 

 white, somewhat pearly. L. 12 ; H. 9 ; W. 7. These dimensions 

 are of the shell resting on its nearly straight base. — New Jersey 

 and north. Not rare in 3-30 fathoms. 



5. Dreissensia, Van B. 

 Shell like Mytilus, but without its pearly lining. 

 Animal with the mantle closed, and with siphon tubes. 



