CULTIVATING PURE STRAINS OF PARAMOECIUM. 187 



solution. From this fact it might be inferred that certain osmotic pres- 

 sures are more favorable than others to rapid division in Paramoecia, 

 and that in the medium composed of hay infusion and tap-water, these 

 favorable osmotic conditions are more nearly approached than in the undi- 

 luted infusion, made with distilled water. 



The practical finding of our experience, then, is that the medium 

 most favorable for cultivating pure strains of Paramoecium in large 

 numbers is composed of standard hay infusion diluted one-half with 

 filtered tap- water; to which mixture is added a small quantity of fresh, 

 wheat bread. 

 FRA^x'Es J. Dunbar, 



Zoological Laboratory, University of Michigan, 



Ann Arbor. 



ON THE VARIATION IN THE JAW OF SUCCINEA 0VALI8. 



BRYANT WALKER, 



(Abstract.) 



The simple form of the shell in the genus Siiccinea, and the great 

 amount of variation in shape exhibited in many of the species, render it 

 extremely- difiticult to draw specific lines clearly between the forms 

 which have been described as distinct. It is therefore desirable to ascer- 

 tain whether the animals afford any reliable specific characters in their 

 anatomical peculiarities. 



This paper gives the result of work undertaken for that purpose, which 

 is yet incomplete, and deals only with the variation in the jaw of a 

 single species. 



The material examined consisted of thirt}- specimens from eight locali- 

 ties in four states, viz : Michigan, 4 ; Indiana, 2 ; Illinois, 1, and North 

 Carolina, 1. 



The special feature of the jaw in this species, which has been men- 

 tioned by previous authors, is a variable number of prominent trans- 

 verse ribs, which denticulate the cutting edge of the jaw. 



This was found to exist to a greater or less extent, in all the speci- 

 mens examined. But the variation in number, size and position of these 

 ribs, was very great. Not only were no two of the jaws alike, but not 

 in a single jaw was the number or position of the ribs on both sides of 

 the central projection, which was uniformly present, the same. There 

 was, however, a great similarity in the jaws from each locality, in color 

 and thickness. 



The result to be deduced from the limited amount of material ex- 

 amined, is that in this species, no reliable specific character can be 

 found in the number and position of the ribs. 

 Bryant Walker, 



Detroit, Mich. 



