NO. i2o.-,. svxtn'srs or rin: XJIADES SIMPSOX. 503 



made of the Unionid;e, and containing more types tliau any other iu 

 existence. In addition to this the U. S. National Museum possesses a 

 large collection, including many types, and for the most part carefully 

 determined by that able student J)r. .lames Lewis, as \vell as the entire 

 Jeffreys collection. A great amount of valuable material has been 

 lately added to it, including the Morelet collection of Naiades, a set of 

 Burmese shells from Fea, a series of New Zealand forms from Suter, a 

 large number of species from von Ihering, from South and Central 

 America and Mexico, the entire series of types of Mr. S. H. and B. H. 

 Wright, and types from many other sources. The writer has personally 

 examined the collections of Say, Conrad, and Katinesque, iu the Acad- 

 emy of Natural Sciences in Philadelphia, as well as the excellent alco- 

 holic series which was the basis of all of Dr. Lea's studies. Through 

 the courtesy of the officers of the New York State Museum of Natural 

 History the entire collection of Naiades of Dr. Gould was loaned to the 

 U. S. National Museum, iu order that he might study it Besides this 

 he has examined the fine collections of Mr. Bryaut Walker, Mrs. George 

 Andrews, Prof. A. G. Wetherby, and a great amount of material 

 belonging to Mr. William A. Marsh, Mr. B. H.Wright, and many 

 others. He has made- critical examinations of the soft parts of more 

 than four hundred species, American and foreign. 



Aside from the careful work of Lea, Trosch^l, and Pelseneer, little 

 has been done in the way of studying the anatomy of the Naiades. The 

 soft parts of a good many foreign species have been examined, and 

 descriptions published which do not describe. Authors have gone into 

 details of the color of the organs and of insignificant characters, but 

 have paid no attention to really important points. 



In my paper on The Classitication and Geographical Distribution 

 of the Pearly Fresh-water Mussels, 1 I placed a great variety of forms 

 under the generic name Unio. Since that time additional knowledge, 

 gained largely from a study of the soft parts, has led me to the belief 

 that it would be best to dismember this genus somewhat as the old 

 group Helix has been dismembered by Pilsbry. Ordinarily the soft 

 parts of most of the Unionida- show but slight differential characters, 

 but at the time when the ovules pass down into the gills a most remark- 

 able change in those organs generally takes place. In the Anodonta 

 edentula of Say, short, horizontal ovisacs are developed, which run 

 directly across the animal, and which at maturity break through the 

 outer walls of the outer gills and pass with their young entire into the 

 water. In the forms typified by Unio anodontoidcx the young are con- 

 tained only iu very distinct vertical or oblique ovisacs in the hinder 

 part of the outer gills; in Unio crassidens, pictoruw, and the like, the 

 embryos fill the entire outer gills, forming thick, smooth pads; in Unio 

 metanevrm, trigonxs, multiplicatiis, and allied forms, they occupy all 

 four of the branchia- throughout. In Unio pliaseolus the smooth outer 



'Proc. U. S. Nut. Mus., XVIII, 189(3, pp. 295-343. 



