420 NEAV GENUS AND SOME NEW SPECIES 



22. U. parvus*, Bar. 



23. U. sesopus, Green. 



24. U. calceolusf, Lea. 



25. U. lanceolatus, Lea. 



26. U. donaciformis, Lea. 



27. U. ellipsis, Lea. 



28. U. irroratus, Lea. 



29. U. lacrymosus, Lea. 



30. U. ater, Lea. 



31. U. rubiginosus, Lea, 



32. U. heterodon, Lea. 



33. U. sulcatus, Lea. 



34. U. planulatus, Lea. 



35. U. circulus. Lea. 



36. U. multiradiatus, Lea. 



37. U. occidens, Lea. 



38. U. securis, Lea. 



* This is rather the smallest species with which I am acquainted. Barnes says 

 it is " the smallest and most beautiful of all the genus yet discovered in America." 

 In this he alludes to the nacre only, which is more pearly and more brilliant than 

 any species I have seen. The exterior presents nothing peculiar but its concentric 

 waves on the beaks, and a slightly elevated rib passing from the beaks to the pos- 

 terior margin. 



t Although I had three specimens of this shell in my possession when I descri- 

 bed it, I felt apprehensive it was too closely allied to the Alasmodonta of Say to be 

 considered as an Unio ; but as a lamellar plate really existed with an incipient 

 tooth, though small, on each valve, besides the large cardinal tooth, I determined 

 it to be the safest plan to class it with the Uniones. I have recently received largor 

 specimens in which this plate almost entirely disappears, while in younger speci- 

 mens it is more evident. 



