No. 434.] 



NOTES ON THE UNIONIDjE. 



along the other margins, but otherwise of various shapes, the 

 dorso-ventral diameter (properly altitude) showing considerable 

 differences. There are no spines, the margins of the valves are 

 simple, or nearly so, and the shell can be entirely closed. There 

 are minor differences in the configuration of the shells and the 

 soft parts. 



The Hinge. — The species of the old genus Unio, with few 

 exceptions, have perfect 1 hinges, and this seems to be a feature 

 characteristic for the whole group, and is of systematic impor- 



In the species ranged under Anodonta, Alasmidonta, etc., on 

 the other hand, the hinges are generally more or less deficient, 

 or wholly wanting. Symphynota pressa Lea {Unio press us) is a 

 real, or apparent, exception, it having rather perfect but feeble 

 hinge teeth. Other features of the shell, the soft parts and the 

 embryos, show relationships with Symphynota, such as S. costata 

 Raf. Natura non facit saltus. 



In this connection should be mentioned the genus Margari- 

 tana, some species and varieties of which have perfect and others 

 imperfect hinges, in adult specimens. 



The muscles, especially the large adductors, are generally of a 

 different color and texture in the Alasmidonta-Anodonta group 

 than in the " Uniones." 



The Marsupia. — It is now generally known that there are 

 two different types of embryo-bearing branchial', or parts of such. 

 In Anodonta, Alasmidonta, etc., and in a part of the Uniones (in 

 the older, wider sense) the ova and embryones are lodged only 

 in the outer, or all four branchiae, which, when not gravid, are 

 of the ordinary formation and appearance. In another group of 

 the Uniones, the marsupia are not only invariably in the outer 

 branchiae, but also permanently differentiated, so that they 



mately fixed numbers of ovisacs for each species, or extending 

 over the outer branchiae through their whole length, as in Ptycho- 



