SO G- O. Sårs. 



anal sinus; but otherwise the figure he gives (a phototype 

 from a mounted specimen) exactly agrees with the form here 

 under consideration. It is also very probable that the form 

 recorded by Mr. Richard from Chili as P. aduncus Jurine, 

 var. is the same species, and that he has confounded it with 

 the European species, on account of the very similar sculp- 

 ture of the shell. The form of the caudal part, however, 

 is very different in these 2 species. 



Biological Observations. — I found this form 

 in only one of my aquaria, but there not unfrequently. The 

 specimens were easily to be observed, owing to their habit 

 of attaching themselves to the walls of the aquarium, more 

 generally in its uppermost part. The swimming movements 

 of the animal are not very rapid, constituting a rather even 

 leap through the water, during which the body is kept in a 

 horizontal attitude, with the back downwards. Though this 

 form was observed for some time, I did not succeed in 

 detecting any male specimens. 



Occurrence. — The aquarium in which this form 

 occurred, was prepared with mud from Argentina. 



Distribution. — The specimens examined by Dr. 

 Vavra, were taken together with Dunhevedia odontoplax from 

 some ditches at Valdivia, Chili; and the form recorded by 

 Mr. Richard as P. aduncus Jur. var. was also from the same 

 country. 



Gen. Euryalona, n. 



Generic Characters. — Carapace large and broad, 

 not very compressed, more or less quadrangular in shape. 

 Valves gaping widely in front and bulging greatly below, 

 lower edges setiferous only in the middle. Head comparatively 



