MOLLUSCA. 465 



gin a short distance below the posterior extremity of the hinge- 

 line. The lower portion of the shell, which comprises the greater 

 part of the surface, is covered with sharply angular and faintly 

 crenulate ribs which curve strongly forward in front, the more 

 posterior ones passing in a nearly straight line from the bounding 

 ridge above to the basal margin, the interspaces between these ribs 

 are broad in front becoming regularly narrower towards the pos- 

 terior portion of the shell. The upper portion of the surface is 

 divided longitudinally by a shallow groove along the lower side of 

 which the ribs of the lower portion of the shell originate ; they pass 

 obliquely backward from their point of origin, bending more or 

 less abruptly downward as they cross the bounding ridge ; above 

 the longitudinal furrow the surface is continuous for a short dis- 

 tance with the general surface of the valve and is then abruptly 

 inflected to the hinge-line to form a rather broad escutcheon; 

 from the upper margin of the longitudinal furrow a series of about 

 10 obscure ribs originates/which are directed obliquely backwards 

 and continue in that direction to the margin of the inflected por- 

 tion of the shell when they bend abruptly forward, becoming much 

 stronger and continuing to the hinge-line which they meet in 

 nearly right angles. Besides the ribs the surface of the shell is 

 covered with obscure concentric lines of growth. 



Remarks. Whitfield's illustration and description of this 

 species are misleading. The type specimen is very imperfect and 

 has the surface much injured, the illustration being greatly re- 

 stored. According to the original description there is no differ- 

 entiation of the upper portion of the shell, but a little further de- 

 velopment of the type specimen has shown the surface features to 

 be as has been described above. The species is a common one in 

 its type locality at Beers Hill cut, south of Keyport, and the recent 

 collections of the Survey contain many specimens which show the 

 essential features of the shell far better than the type. 



Formation and locality. Tinton beds, Beers Hill cut, south of 

 Keyport (i29 5 , I29 7 , 129), near Freehold (132). 



Geographic distribution. New Jersey. 



30 PAL 



