MIOCENE MOLLUSCA AND CRUSTACEA. 33 



as differing from the normal type "in the greater elevation of the ears and 

 tlie more distinct quadraugulation of the nbs of the convex valve." As 

 regards the elevation of the ears, they both appear to have the hinge line 

 on a level with the point of the beak; and as to the extension laterally of 

 the ears, I should judge from Mr. Conrad's figures that they were closely 

 similar; although I have never seen a complete valve from New Jersej', 

 there are some with perfect ears, and I can see no impoi-tant difference 

 between these and the figures given by Mr. Conrad. The difference in the 

 form of the ribs, if we can rely on Conrad's figures, is quite a marked one, 

 as the New Jersey specimens have the furrows deep and the edges 

 extremely abrupt on the convex valves. 



The New Jersey examples seem to have attained a much larger size 

 than those from Maryland. I have before me imperfect individuals which 

 measure almost 3 inches across the ears, and of which the valves must have 

 measured over 5 inches in width when perfect. The general form has been 

 nearly equilateral and orbicular in outline, with large ears on each valve; 

 one valve nearly flat or but slightly convex, the other moderately convex 

 The flat valve has the ears de})ressed below the general level and a consid- 

 erable space around or below the beak concave and somewhat depressed, 

 but not to a level of the surface of the ears. Each lateral margin forms a 

 broad convex rib which is longitudinally striated; besides these, there are 

 seven sharply elevated, prominent, and narrow ribs, a central <ine and three 

 on each side; the surface of the ears is also obscurely radiated, while the 

 entire surface is marked by fine, concentric, lamellose lines, strongh' ele- 

 vated. The convex valve has three broad very slightly convex i-ibs on each 

 side of the central groove and a broad flattened space outside of these. 

 The ears are slightly convex on their surfaces and are distinctly separated 

 from the body of the shell by a depressed line. The right ear, when the 

 shell is held with the convex side up, has a moderate constriction above the 

 body of the shell, and the opposite one is very slightly extended along the 

 shell in the same place. The entire surface of the valve, ears, lateral 

 borders, and ribs is finely striated longitudinally and also concentrically. 

 Tn the interior of the valves the ribs are profoundly marked, on the flat 

 valve by deep narrow grooves and in the convex valve by broad rounded 



MON XXIV 3 



