INVERTEBRATE PALEONTOLOGY. 513 



lie based the distinctions, however, almost entirely on anatomical characters, 

 observed in the soft parts of the animal. At a later date, Dr. Lea, of Phila- 

 delphia, who has given more attention to the study of this family of mollusks 

 than any other person, after dissecting the animals of many of the same spe- 

 cies, as well as of others, maintains that the characters given by Professor 

 Agassiz are not constant, sometimes even in different individuals of the same 

 species, and therefore argues that they cannot be relied upon in distinguish- 

 ing genera in this family. However this may be, it must lie evident that 

 characters ot this kind, unless accompanied by some visible peculiarities ot 

 the shell, are of little or no use to the palaeontologist in classifying fossil 

 species. 



For the above-mentioned reasons, I have preferred to define this genus 

 here in its more comprehensive sense. Many of the names, however, quoted 

 in the synonymy, may be conveniently used for sections or subgenera, even 

 in palaeontology ; that is, such of them as can be distinguished by characters 

 of the shell alone. 



Widely different views in regard to the subgenera that should be 

 admitted under this genus are maintained, even among the most respectable 

 authorities, who have devoted much time and study to the group. Dr. Lea, 

 of Philadelphia, has proposed to reduce Unio proper to the rank of a subgenus, 

 under a comprehensive genus, to which he applies the name Margaron, 

 including the following groups as subgenera, viz, Triquetra, Prisodon, Unio, 

 Margaritana, Plagiodon, Monocondi/hea, Dipsais, Anodonta, Columba, and 

 Bysmnodontu ; while many other authorities regard these names as repre- 

 senting so many distinct genera. 



H. and A. Adams, in their work on the Genera of Recent Mollusca, 

 admit a number of subgenera under the genus Unio; and, although not 

 entirely satisfied that their arrangement is the best that might be devised, I 

 think it will be found as convenient as any yet proposed by those who view 

 the sections they include as representing a single genus. Their arrange- 

 ment is as follows, viewing the first or typical section as represented by U 

 pictoru?n, Linn, (sp.) : 



2. bariosta, Raf. (— Potamida, Swainson). 



Shell not winged, arcuated ; surface of valves smooth. — ( Unio 

 emarginatus, Lea.) 



05 H 



