300 THE PEARLY FBESH- WATER MUSSELS—SIMPSOX. vol. xvm. 



Genus UNIO, Retzius. 



Tt seems to be impossible to ascertaiu with certainty avIio is tbe 

 author of this genus. In 1788 Laurentius Miinter Philipssou described 

 it in a thesis delivered under the presidency of his master, Retzius.^ 

 at the University of Lund, in Sweden, at a public examination for a 

 doctor's degree. Whetlier Philipsson or Retzius should be credited 

 with the genus can not be positively known, as it is believed by some 

 that the master was the author of the dissertation, which the student 

 merely defended. I am inclined to take this view of the matter, for the 

 reason that Retzius was an author of repute, while it is not known that 

 Philipsson ever gave any attention to natural history or was the author 

 of any genera or species before or since. There was no special desig- 

 nation of any type, but the species were mentioned in the following- 

 order: Uniomargaritifervs, U.crassus, U.tnmidus, U.pictorum, U.ovalis, 

 and V. corruf/atus. 



We can not consider the genus Manjaritana, founded on the absence 

 of lateral teeth, a valid one, because the first species which is mentioned 

 in this list is the type of the genus U)iio (and also of Margaritana, 

 founded many years later), and this is placed by itself in a section 

 which is designated as lacking lamellar teeth,- while the other five spe- 

 cies are put in a second section, characterized by lateral teeth. There- 

 fore, in case of a generic separation, founded on the presence or absence 

 of lamellar teeth, the species wanting them avouUI have to be j)laced in 

 the genus Uirio, and another name given to the forms having both sets 

 of teeth. But, as I shall show farther on — I think satisfactorily — that 

 the different species usually placed in Margaritanu are merely Unios 

 with ordinarily imperfect teeth, we can use Retzius' generic name to 

 include all the forms that are commonly placed in the two genera. 



The genus Unio is by far the most numerous in species, and is the 

 most widely distributed of any of the Naiades, as well as the most 

 variable in its characters. It is found in the fresh waters of all the 

 continents, especially in the rivers and streams, while the nearly related 

 Anodonta is more commonly an inhabitant of lakes and ponds. 



In the Bast Indian Archipelago it is met with in perhaps all the larger 

 islands, extending east into the Solomon group; it is abundant in Aus- 

 tralia, New Zealand, Tasmania, the Philippines and Ja[)an. It is 

 found in Ceylon. Madagascar, the British Isles, and in Cuba. The only 

 considerable continental areas in which it is believed not to occur are 

 that part of North America lying south of the fortieth parallel of north 

 latitude, having a drainage into the Pacific; the extreme arctic regions, 

 and a considerable area of the Sahara and Gobi deserts. 



'Dissertatio historico-naturalis, sistens nova testaceorum geuera, p. 16. The fol- 

 lowing is the original diagnosis: "Unio. — Animal ascidia. Testa bivalvis, jcqui- 

 valvis, ajquilatera. — Cardo. Dens ani in valvula destra solidus, subiutrusus, in 

 sinistra duplex; omnes crenulati. In pliiiimus dens vulva' lougitudiualis lamel- 

 laris intra siuistr* valvule bilamellarem. 



- Dente vulvie uullo, sed margo horizoiitalis. 



