THE NAUTILUS. 93 



heavy in crassideng, and the teetli very strong and of the same type 

 with the other species of group B, while the decided downward 

 curvature at the posterior end in old specimens approaches both 

 species to the same, and removes them decidedly from A. 



In U. multiplicalus the female was found November 1, with evi- 

 dently mature embryones filling the uterus sacs, which are of rather 

 the same appearance as those of U. alaius, and occupying the most 

 posterior part of the outer branchiae. Also, otherwise this species is 

 quite different from U. undulatus Barn., in spite of the similar ap- 

 pearance of the old mussels, as already pointed out by Say. 



A few words about U. alatus Say, laevissimus Lea and gracilis 

 Barn, may be added. Of all three the branchial uteri were found 

 filled late in October, in the former evidently discharging the em- 

 bryones, and of a rather different formation from that in the two 

 others, while the young mussels of the two former species are very 

 much alike. It is known that in U. alatus the female mussel is 

 markedly produced downward, and more so in giaeilis, while in 

 laevissimus it is scarcely distinct from the male. 



Besides these two main, and, as it seems, most numerous groups, 

 there are some species of quite different types. In order not to let 

 this article become too lengthy, they, together with Murgnrifuna and 

 An.odonta will be considered later. 



It is probably known to most conchologisls that during the first 

 two or three vears of life, the genital glands are not developed at 

 all. Yet the specimens are still rather small and young when they 

 begin producing ova and spernia.and the assertion that the presence 

 of embryones is a criterion of maturity of the parent is as far from 

 truth here as it is for the Cyeladidce. In group A the filled uterus- 

 or embryo-sacs increase in numbers as well as in size with advancing 

 age of the mother animal. In U. subovatus Lea, e.g., 18, 25,28, 40 

 have been counted on a side, in specimens of different sizes, and 

 these are probably not the lowest and highest numbers to be found, 

 and correspondingly so in other species. For every one of them 

 there is, however, an average number the rule for an adult, and 

 these numbers are very different for the different species. 



The embryones, or glochidia, of many species have been exam- 

 ined and figured long ago, especially by Lea, and they prove to be 

 of rather different forms. Very probably they would be found more 

 or less uniform in the several groups, and with the latter show 

 differences which may also be of value for systematic^. Although 



