THE NAUTILUS. 19 



very interesting statements regarding their anatomy. He places 

 those species in which the young are carried in the posterior part of 

 the outer branchiae, and the female shell is inflated in the post-basal 

 region, in a group designated as A, which he states, as a rule, have 

 bright shells, and are gravid from late summer to winter. In group 

 B all four branchiae are charged throughout their length ; the shells 

 are generally dull colored, and do not show marked differences be- 

 tween male and female. 



In my earlier attempts at classifying the Naiades, I based my ar- 

 rangement almost wholly on shell characters. I did this because in 

 the splendid collections of the National Museum we had either the 

 types or authentic specimens of a very large proportion of the 

 known, valid species, and because I was anxious to see whether a 

 classification could be based on the shells alone that would be sup- 

 ported by the evidence of the anatomy. I have since then ex- 

 amined the soft parts of a large number of species, and carefully 

 tabulated the results of the work of Lea and others, and I am ex- 

 ceedingly gratified in being able to say that in almost every case the 

 characters of the animal and shell seem to essentially agree. In a 

 few instances, the evidence of the former has thrown important light 

 on relationships which could not be determined with certainty from 

 shell characters alone, and vice versa. In some cases, where there at 

 first seemed to be a contradiction, more careful study has shown es- 

 sential agreement. 



There are certainly two great groups of North American Unios. 

 In the first the shell is generally, though not always, covered with a 

 hard, smooth, bright epidermis, which is often rayed or marked 

 with patterns of attractive color. It is rarely sculptured with any- 

 thing beyond slight concentric ridges, and in only a few instances 

 has it any ridge on the posterior slope, and, with possibly one or two 

 exceptions, the outline is never arcuate, even in old age. The fe- 

 male shell is usually decidedly swollen in the posterior basal region 

 to accommodate that part of the outer branchia? which contains the 

 embryos. The beak sculpture is generally delicate, consisting, for 

 the most part, of close, fine, paralled ridges, which have a tendency 

 to fall into an anterior and posterior loop, the latter sharp pointed 

 below. 



The embryos are contained in the posterior part of the outer 

 branchiaB. The ovisacks are distinct, being separated by a suture, 

 and the whole marsupium is rounded below, projecting generall 



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