No. 434-] 



NOTES ON THE UNIONIDJS. 



1 1 1 



actual observation, e.g., on L. vcntricostts. Whenever any larger 

 object touches the margin of the mantle, that is the long and 

 partly branched fringes, they at once move inward and by inter- 

 lacing form a fine network. Some other species also, e.g.. C. 

 irrorata, with the valves not opening wide, have the papilla? only 

 moderately developed. 



The systematic position of Margaritana is still uncertain. In 

 shape and appearance of the shell, and nacre, they resemble more 

 Unio, than Alasmidonta. The hinge is perfect in some and 

 defective in other species, in what seem to be individuals, typic- 

 ally perfect, and in younger specimens. It has been mentioned 

 that Gysser occasionally found all four branchiae charged with 

 ova, and in all probability they are summer breeders, like Unio, 

 etc. That, however, must be ascertained by actual observation. 

 The crucial test will be found in the examination of the em- 

 bryos. 1 — I have also seen in some specimens of Ouadntla kirt- 



scattered over the inner surfaces of the valves inside the pallial 

 line, described as characteristic for Margaritana. 



The second main division consists of Alasmidonta, Anodonta 

 and their allies, and these two genera seem to represent the two 

 main groups. Among the former. Stroplntus cdcntnlu^ Say. 



generic rank. As regards the shell, soft parts and formation of 

 embryos, it stands very near to Alasmidonta, but the ova and 

 embryos are imbedded, in various numbers, about from ten to 

 twenty, in cylindrical albuminous masses placed transversally in 

 the outer gills. In my opinion, these masses, which have also 

 been compared with and called placentae, are not homologous 

 and equivalent with the ovisacs of Lampsilis, etc. The question 

 deserves more study and comparison. 



Some other species and genera, c. g.. I.astcna lata Rat. nee 



teristics and their exac 

 The following table 



