24 THE NAUTILUS. 
The third paper mentioned above is already so condensed that 
any abstract is difficult to make. It deals with the questions of 
classifications, mutual relationships of the genera, and geographic 
distribution. Margaritana is merged in Unio, as the species are 
believed to have lost the lateral teeth by degeneration or disease. 
It is an error, however, to give U. margaritifer (not “margaritiferus”’ ) 
as type of Unio; and it should be noted that in having a series of 
small muscle-sears in the middle of the disk, M. margaritifera, mon- 
odonta, ete., differ from any Unios as well as from the M. undulata, 
rugosa group, which is more properly called Alasmodonta. These 
groups seem to be as valid genera as Castalina, which Simpson re- 
tains. Simpson, in common with other recent authors, recognizes 
two families, Unionide and Mutelide. The former containing gen- 
era Unio, Anodonta, Prisodon, Tetraplodon, Castalina, Burtonia, 
Arconaia, Cristaria, Lepidodesma, (new genus for U. languilati of 
China), Pseudodon, Leguminaia, Solenaia.  Mutelide contains 
Mutela, Chelidonopsis, Spatha, Pleiodon, Brazzea, Glabaris, [hering- 
ella, Monocondylea, Fossula, Mycetopoda. Each genus is discussed 
in a separate paragraph, and a careful reading inclines us to place 
great reliance upon Mr. Simpson’s conclusions. <A full synonymy 
of each genus would have been a useful addition, for there are some 
generic names not mentioned in the text, probably because they 
prove to be mere synonyms. 
The geographic provinces indicated by Unionide and Mutelide 
are: Palearctic, including all Europe, Africa north of Sahara (ex- 
cept the Nile), all northern Asia, and the Pacific drainage of North 
America. Ethiopian. Oriental, including southern Asia to Japan, 
Philippines, Malay Archipelago and to the Solomon Is. Australian, 
Australia, Tasmania and New Zealand. Mississippian, the Gulf 
drainage, spreading to N. C. and Central America. This region is 
the richest in species in the world. Atlantic, Atlantic drainage from 
Florida to Labrador. Neotropical, the whole of South America. 
Central American, Panama to Mexico and Cuba. A map presents 
the areas of distribution graphically. In conclusion Mr. Simpson 
discusses the geological history of the groups. Students of the 
Uniones will find this paper crowded with important facts and care- 
ful reasoning from them, in the main very reliable. Our experience 
with other groups leads us to believe, however, that more import- 
ant points will follow an anatomical study of the Uniones than have 
yet been developed. 
