63 



ARTICLE V. 



Observations on the Naiades, and Descriptions of New Species of that and 

 other Families. By Isaac Lea. Read before the American Philo- 

 sophical Society.) May 7, 1S30. 



SINCE I presented my last paper on the Naiades, which was publish- 

 ed in the third volume of the Society's Transactions, 1 have been fortu- 

 nate in obtaining many species hitherto undescribed. 



I have possessed for several years many individual specimens which 

 I supposed to be new, but which I deferred describing until I should 

 possess of each kind individuals of different ages. My late acquisitions 

 have converted my suppositions into certainty ; and I now offer the 

 following descriptions with a view to their publication in the fourth 

 volume of the Society's Transactions, to be accompanied by figures 

 executed like those attached to my last memoir. 



In that memoir I took occasion to make some remarks on the " ele- 

 vations on the surface of the disks." I had not at that time satisfied 

 myself entirely in regard to the manner in which these were accom- 

 plished by the animal. Some fine specimens of the Unio cornutus, 

 in all the stages of growth, having since been procured, I have been 

 able to trace these formations through every degree. 



It will be observed on examination, that the horns alternate j that is, 

 those on one valve are not placed opposite to those on the other; 

 consequently one is made at a time. The animal deposits the secreted 

 carbonate of lime on the outside of the edge of one valve, where the 

 horn is to be formed, and on the inside of the edge of the other ; the 



