502 PROCEEIUXHX i>r mi: x.trfox.iL MTSKTM. V..L.XXH. 



he was led into several serious errors, his work lias proved an important 

 aid in classification. 



H. and A. Adams followed with a classification in 1S57, in which they 

 recognized the families Unionida' and Mutelida-, dividing the former into 

 two subfamilies, Unionime and Mycetopina-, and giving a list of the 

 then known species. 



Agassiz, in 1852, published a classification of the Xorth American 

 Unionid;e, based more especially ou the anatomy, but much of his 

 work was hasty and careless, and as a result his groups are, for the 

 most part, heterogeneous assemblages of unrelated species. 



In 181)3, von Ihering published his Najaden von San Paulo, in which 

 he showed that while certain of the Naiades began their existence on 

 hatching from the egg as a glochi<Uu>n, with a bivalve shell capable of 

 containing the soft parts, others developed into a J*i<linm, with three 

 segments, the middle oue alone having a single shell. He showed that 

 the presence or absence of siphons by which the Mutelida' had been 

 separated from the Unionida' was not a constant character, even in indi- 

 vidual species, .and placed the genera which he believed to have glo- 

 chidia in the family Unionida', and those with lasidia in the Mutelida'. 

 I consider this the most important discovery that has yet been made in 

 the study of the Xaiades. 



In 1896 the writer published a classification of the Xaiades, based 

 largely on his studies of shell characters. In that it was shown that 

 throughout one great group all the genera had shells with schizodont 

 hinge teeth or vestiges of them, and in the rest, a smaller group, they 

 all had taxodont teeth, or vestiges of them. As all the genera whose 

 shells have schizodont teeth were believed by von Iheriiig to possess a 

 glochidium, and those with taxodont teeth have a laaidium^it seemed 

 as though there was a natural division of the Xaiadca into two families 

 Union id <ie and Mutelida-. 



Some years ago the Eev. and Mrs. L. T. Chamberlain, son-in-law and 

 daughter of Dr. Lea, suggested to Mr. Dall that a new edition of the 

 Synopsis, or if necessary a new work, should be prepared, and for this 

 purpose they generously offered to contribute such sums as might be 

 needed for the purchase of material to add to the great collection left 

 by Dr. Lea to the U. S. National Museum at his death, and for li'erature 

 that might be required in preparing the work. As the writer had made 

 a special study of the Xaiades, the task of preparing the new Synopsis 

 fell to him. Since then Mrs. Chamberlain has been called away by 

 death, greatly regretted, but her husband has continued to cheerfully 

 furnish all the needed funds for carrying on the work to completion. 



While the labor of preparing the synopsis has been arduous, and 

 while it has been impossible to obtain material needed in many impor- 

 tant groups, or all the necessary literature, I feel that my opportuni- 

 ties for study have been exceptionally good. I have had constant 

 access to the great collection of Dr. Lea, the finest, no doubt, ever 



