528 UNIO 
whole length of the outer gills only, forming a thick, smooth 
pad when filled with young; gills united to the mantle behind 
to their extreme points, or very nearly so; branchial and anal 
openings unbranched ; superanal opening always closed below. 
Type, Unio tumidus Retzius. 
The above generic description applies to the great majority 
of the species I have retained under the name Unio. 
The genus was described in a thesis by Laurentius Munter 
Philipsson under his master, Retzius, in the University of 
Lund, Sweden, and it is often credited to the former. I am 
informed by Professor Joh. Chr. Moberg, of Lund, that by a 
former law or custom of the university the professor was con- 
sidered the author of all papers which a student under him 
defended. According to this, Retzius must be credited with the 
genus. This law was repealed in Lund in 1852. 
Until recently all the species of the family Unionide having 
nearly or quite perfect pseudocardinal and lateral teeth were 
placed in this great group by most conchologists regardless of 
the form or details of structure of the shell or the characters 
of the animal. Other species in which the lateral teeth were 
not quite fully developed, were placed in Margaritana by some 
writers and in Unio by others. After a careful study of the 
obvious anatomy of a large number of these forms and minor 
shell characters, which seemed-to correspond with anatomical 
peculiarities, I became convinced that it would be better to dis- 
member Retzius’ genus and did so in the Synopsis. As there 
restricted it is still the largest generic group contained in either 
of the two families treated, and I have given it practically the 
same limits in this work. The forms, which have all four 
leaves of the branchiz closely filled with embryos, seem to me to 
belong to a somewhat different phylum from those in which 
only the outer gills are filled. If von Ihering is correct in as- 
suming that the Naiades, which have shells with radial beak 
sculpture and embryos in the inner gills alone, are the primi- 
tive forms, then it would seem reasonable that those that con- 
tain them in all four gills represented a step higher in the 
development of these forms, the highest type of Unione life 
