16 PROCEEDINGS OF THE ACADEMY OF 



giL ; alate expansion gradually flattened ; anterior end abruptly rounded, exca- 

 vated by a lunuliform area in front of the beaks ; posterior end rounded below, 

 truncate above. Surface covered by fine, rather regular, sharp, concentric 

 striae ; the alate expansion bears also faint traces of two or three furrows 

 diverging from behind the beak. 



Length -31 (100) ; height -22 (71) ; thickness of right valve -10 (32) ; angle 

 between hinge-line and umbonal ridge 35. 



This little species closely resembles A. arguta, Phill., sp., var. de Kon., 

 ( Auim. Foss., pi. iii. fig. 1 2.) The beaks, however, are still more terminal than 

 in that variety, and the lunette is not so sharply bounded. Area arguta, var., 

 is from carboniferous limestone Vise. Though the dental characters of the 

 present species have not been observed, it can scarcely prove to be anything 

 but an Area or Macrodon. 



MACRODON, Lycett. 



Macrodon cochlearis, n. sp. Shell of moderate size, rather ventricose, 

 length equal to twice the height ; beaks subterminal, flattened, incurved. 

 Ventral border straight, or slightly sinuate in the middle, symmetrically 

 curved toward the extremities ; posterior extremity truncate from the hinge- 

 line one- third its width ; anterior extremity most projecting above the mid- 

 dle, excavated above by a small deep lunette ; dorsal line straight, nearly a? 

 long as the shell, not inflected ; posterior cardinal extension with feeble in- 

 dications of one or more lateral teeth. Surface finely striated concen- 

 trically. 



Length -88 (100) ; height -44 (50) ; length of anterior end 46 (19) ; of 

 posterior end -72 (81) ; thickness of left valve '18 (22). 



Resembles M. parvus, White and Whitefield, but, besides its larger size, it 

 is much less ventricose, especially in the posterior half, and has not the 

 conspicuous muscular pits of that species. 



NtTCULA, Lamarck. 



Npcula microdonta, n. sp. Shell small, transversely oblong ; height 

 equal to two-thirds the length ; beaks small, somewhat incurved, but little 

 elevated above the hinge-line, about one-third the length from the short end. 

 Ventral border rapidly curved, and regularly so to the vicinity of the long 

 end, where it is slightly sinuated, from which point a shallow groove extends 

 up nearly to the beak. Dental plates but little angulated between the beaks ; 

 the larger bearing near its oiiter margin 10 or 12 minute transversely tuber- 

 cular teeth, and the shorter 4 or 5. Teeth not distinguishable to the beaks, 

 but no cartilage pit seems to be present. Anterior muscular pit oblong, sur- 

 mounted by a large pedal scar. Shell most ventricose in the middle. No 

 surface markings discernible. 



Length -47 (100) ; height -32 (68) ; length of short end -18 (38) ; of long 

 end -29 (62) ; thickness of both valves -18 (38). 



This little species is readily distinguished from N. Iowensis, and most 

 others of this age, by its very small teeth and the absence of the usual 

 nuculoid outline. 



Ranges from bed No. 5 into the base of the Burlington limestone. 



LEDA, Schumacher. 



Leda saccata, n. sp. Shell very small, transversely elongate, rostrate at 

 the longer extremity ; obtuse, ventricose and saccate at the other. Beak 

 abruptly, though moderately drawn out, and but slightly incurved. Ventral 

 side strongly curved, becoming nearly straight toward the rostral extremity. 

 Dorsal region deeply excavated for an escutcheon on the longer side of the 

 beak ; hinge plates bearing each six or seven teeth. Greatest thickness of shell 

 between the beaks and the middle. Pit of adductor of short end very deep 



[Jan. 



