1903.] NATURAL SCIENCES OF PHILADELPHIA. 265 



The hinge-line is straight externally, broadly feebly arcuate internally, 

 the line of teeth more than three-fourths as long as the shell, the lateral 

 teeth becoming longer and strongly oblique. The space between 

 the beaks and the hinge-line in flattened, nearly smooth except some 

 fine, close-set parallel hnes of growth, but at the posterior end there 

 are some coarser parallel and feebly oblique lines. The radial ribs 

 are 28 to 31 in number, rather coarse and separated by much less than 

 their own widths, except in the feebly depressed area radiating from 

 the middle of the beaks where they become finer and relatively much 

 more widely separated, and generally with one fine intermediate rib 

 between them in this region toward the ventral margin only ; the ribs 

 also become smaller but very close-set posteriorly in the flattened area 

 toward the hinge-line. The surface posteriorly at an angle of about 

 30 degrees with the hinge-line is convex, becoming rapidly declivous 

 and explanate to the latter. The muscular scars are rather deep. 

 Lines of growth produce feeble transverse and rather widely separated 

 nodules on the ribs generally becoming obsolete posterior^. The 

 length of a moderately large individual of this species is 11.5 mm., the 

 height 6 mm. 

 Area delicatula n. sp. 



Occurs in the Lower Vicksburg limestone in great abundance. 

 It may be regarded as a homologue of invidiosa and is doubtless one 

 of the smallest known members of the family. It is elongate, very 

 inequilateral, obliquely parallelogramic, moderately inflated, becoming 

 flattened posteriorly toward the hinge-line, the latter long, thin and 

 straight, the teeth small. The space between the hinge-line and the 

 beaks rather low, flat and smooth or nearly so, narrowing very 

 gradually posteriorly. The umbonal impression, with its diminished 

 ribs, is nearly as in invidiosa and many other species. The ribs are 

 some 28 in number, relatively moderately coarse, being generally sepa- 

 rated by nearly their own wddths, flattened. Length of a moderately 

 large valve 6 mm., height 2.6-2.8 mm. 



It is somewhat singular that no reference has been made to the very 

 different sculpture of the right and left valves of Area lesueuri Dall 

 (mississippiensis Con.). The left valve has the diverging ribs double, 

 the pairs being much more close-set than the single and smaller ribs 

 of the right valve. It results from this that the left valve is much the 

 stronger and more frequently preserved intact. 



Area vaughani n. sp. 



While mentioning the genus Area, it may be appropriate to allude 

 to a species, quite common in the Lower Claiborne at St. Maurice, 



