1)1(5 PROCEEDINGS OF THE ACADEMY OF [August, 



1. Lateral teeth with the longer denticles very long; median 



teeth broad, with the denticles in one transverse series, 



P. " lordi" (New Mexico). 



Lateral teeth with the larger denticles short, median teeth 



with the denticles in three groups, .... P. gyrina. 



2. Lateral teeth with the denticles seven in number (counting 



the apical one), straight or nearly so; median teeth with 



the denticles in three groups, .... P. rhomboidea. 



Lateral teeth w^ith the denticles five in number, very oblique; 



median teeth with the denticles rather long, in one group, 



P. virgata. 



Individual teeth of any species may vary from the normal type, 

 but it is easy to determine the normal character. 



Figs. 1 and 2 show the frequency polygons for the fossil (Arroyo 

 Pecos) and recent (Gallinas river) series, based on the relation of 

 the length of the shell to the breadth — the most obviously variable 

 character. It is found that the mode, or point of greatest fre- 

 quency, is the same in both, but the skew is toward greater breadth 

 in the fossils, toward narrowness in the recent shells. It is con- 

 cluded that all the shells belong to one species, which has been 

 evolving from a broader to a narrower form, perhaps in consequence 

 of a decreasing salinity or flow of the water. Plate XXVI shows 

 a series of each type, including all the recognizable variations. 



The work represented by this paper has been done in the biologi- 

 cal laboratory of the Normal University of New Mexico, under the 

 direction of Mr. T. D. A. Cockerell, who has also revised the 

 manuscript. 



