512 I'i;<>CEi;i>f\<;X OF THE X.lTfOX.lL .1/r.v^r.V. VOL.XXII. 



types up to the highest, most recent, and most complicated, and stand 

 as a sort of index to the progress of the family in the past. 



I have not ben able to study the Mutelida 1 as carefully as 1 have the 

 Unionida'. All the species are foreign, and while I have seen the soft 

 parts of a few South American for-ns, I have never had the opportunity 

 to examine the anatomy of a single African species. I have not been 

 able, from what knowledge I have obtained, to discover any consider- 

 able anatomical or conchological differences between the Mutelid genera 

 of Africa and South 'America which might be used to separate the 

 family into subfamilies. 



I have treated the families of the Unionida- and Mutelida? together in 

 this work because they have both been classed as Xaiades. But the 

 remarkable differences in the embryos, that of the former family being 

 a glochidium with a bivalve shell inclosing the soft parts, and that of 

 the latter a lasidium divided into three segments with a single shell 

 on its middle section, and the shells of the one family having schi/o- 

 dont teeth, while those of the other have taxodout teeth, seem to show 

 that the two great groups are not very nearly related. 



In studyingthe Naiades I have been greatly perplexed on accountof the 

 unsatisfactory and confused condition of much of the literature. Earlyin 

 this century Bafinesque collected the Unionida' extensively in Kentucky 

 and published a large number of genera, minor groups, and species. 



It is probable that there will always be differences of opinion with 

 regard to his work, just as there have been in the past. His figures are 

 more like those made by children, or the caricatures drawn by aboriginal 

 tribes, than the creations of an intelligent naturalist, and the descrip- 

 tions are too brief in many cases to give any clear idea of the species. 

 The work in the continuation of his monograph is even worse than that 

 in the body of the paper, and tribes, genera, and subgenera are mingled 

 in bewildering confusion, and all these are placed under two great genera. 

 It is impossible in many cases to tell what his meaning is. I have care- 

 fully gone over his so-called types in the Academy of Natural Sciences 

 in Philadelphia, making notes and sketches, and stating the names 

 by which they are commonly known. Four times, separated by long 

 intervals, I have studied his original descriptions, with specimens of 

 the same species as these so-called types, in an attempt to determine 

 his species, each time writing down my conclusions, and I believe that 

 quite a number of his species will have to stand. There can be no doubt 

 whatever that many of his so-called types are not types at all, as they 

 do not fit the descriptions by any stretch of imagination. 



In applying generic, subgeneric, and sectional names I have used 

 those which have been previously applied wherever it has been possible 

 to do so by the process of elimination, in order that no claim of priority 

 might be urged for them. 



A most unfortunate dispute arose among our earlier couchologists in 

 regard to priority of names, one in which a great deal of ill feeling 

 was displayed. I have endeavored wherever it was possible to ascer- 

 tain the exact date of publication of these disputed names and to 

 credit the species to the earliest described. In some cases it has been 





