THE NAUTILUS. 81 



This classification is not founded on characters of the marsupia, 

 for he has placed in Quadrulinre groups in which the embryos occupy- 

 all four of the gills, others in which they fill only the outer gills, and 

 still others where they are confined to the hinder or the median part 

 of the outer gills. Besides, he distinctly states on page 39 that a 

 systematic arrangement of the marsupia does not coincide with a 

 natural arrangement of the family. 



Nor do I see how such an arrangement can be based on shell char- 

 acters. Obovaria, which is placed with Quadrula, has more or less 

 perfectly developed dimorphism in the shells. In 0. ellipsis the 

 female shells are almost always swollen at the posterior base, and the 

 same is the case with 0. lens and 0. circt/lus, while in 0. castaneus the 

 male and female shells are as distinct as in any species of Lampsilis. 

 Ptychobranchus, with its wonderfully folded marsupium occupying 

 the entire outer gills, with the shells of male and female alike, he 

 places in the genus Lampsilis. Why he does so I do not know, as 

 the group differs most decidedly from Lampsilis in the characters of 

 shell, marsupium and animal. 



On the other hand, he places Truncilla personuta Say, T. perplexa 

 Lea, and T. foliata Hild., in the genus Quadrula! To me such an 

 arrangement is absolutely astonishing ! The male shell of the first- 

 named species is somewhat triangular, and does resemble a Quadrula 

 somewhat. The female shell is very different, being quadrate and 

 having a decided, gaping, toothed post-basal swelling. In T. per- 

 plexa the female shell has a great rounded post-basal swelling, which 

 differs in thickness, texture and color from the rest of the shell. I 

 do not think there is a species known in Avhich the differences be- 

 tween the male and female shells are so great as they are in T. 

 foliata. In the male shell at the place where there is a compressed, 

 radial, central area the outline of the female is carried down into an 

 enormous and elongated, rounded wing. That these should belong 

 in a genus in which the shells of male and female are alike and from 

 which the animal and marsupium so widely differ, is beyond my com- 

 prehension ! If such an arrangement is a natural one then I am sure 

 that all the years I have spent in patiently and lovingly studying the 

 Naiades have been absolutely wasted. It seems to me that we might 

 just as well go back to the arrangement temporarily adopted by Dr. 

 Lea, of grouping together in one lot those forms which have a wing, 

 and in another those which have none, and subdividing these groups 

 into small ones founded on form and sculpture of the shell. 



