MOLLUSCA. 631 



Super-family MACTRACEA. 

 Family MACTRIDAE. 



Genus Mactra Linneus. 



Mactra pentangularis n. sp. 

 Plate LXXI., Figs. 7-8. 



Description. — Shell subpentagonal in outline, the dimensions 

 of the best type specimen being- : height, 17 mm. ; length, 18 mm. ; 

 convexity, 5 mm. ; another somewhat distorted example is pro- 

 portionally longer. Beaks subcentral, the cardinal margins 

 sloping away on either side at an angle of about 122° ; anterior 

 and posterior margins both nearly vertically subtruncate, the 

 anterior margin being slightly more convex, and both rounding 

 below into the gently convex basal margin; greatest convexity 

 of the valves on the median line above the middle, the surface 

 sloping almost equally tO' the anterior and posterior margins, 

 with a very slight posterior unibonal ridge. Surface marked 

 only by fine, concentric lines of growth which become some- 

 what stronger and more crowded towards the margin. 



Remarks. — The hinge characters of this species are not clearly 

 shown in the casts irom which it has been described, but there is 

 an elongate lateral tooth in front of the beaks certainly, and ap- 

 parently also' behind; the characters beneath the beak have not 

 been seen at all. The species differs from other Mactridae in the 

 New Jersey faunas in the proportionally greater height of the 

 shell and its more nearly equal height and length. The species 

 resembles M. nitiditla Meek^. 



Formation and locality. — Woodbury clay, Lorillard (102). 



Geographic distribution. — New Jersey. 



'Rep. on Cret. and Ter. Inv. Foss, of Up. Mo., p. 211, pi. 30, figs. 6 a. 



