158 Annals of the Carnegie Museum. 



agitation of the water occurring in the streams, as is shown by the 

 well preserved condition in which they are collected. While the 

 water is colder, it is clearer; there is a more even temperature. 

 Streams in summer may nearly dry up, or in winter become solidly 

 frozen. Such conditions do not occur in Lake Erie. The effect of 

 this environment is shown in the regular growth lines of the shells and 

 their brighter colors. On the other hand the waters of the Upper 

 Ohio Drainage are usually warmer, contain less lime, and the food 

 conditions are less stable, if even at times food is more abundant. 



V. The accoriipanying table giving the list of localities at which 

 collections were made, will give some idea of the distribution of the 

 species employed in this research. 



VI. Method of Measurement. 



A careful selection was first made of the material on hand to elim- 

 inate all stunted or otherwise abnormal specimens. The following 

 data were then recorded from usually the right valve of each animal: 



1. Length = L. With vernier caliper. (Scientific Materials Com- 



pany Catalog No. 3930.) 



2. Dorso-ventral diameter = D.V.D. With vernier caliper. 



3. Dextro-sinistral = D.S.D. With vernier caliper. 



4 and 5. Distances anterior; (AD), and posterior, {PD, from DVD 

 to extremities of valve, (the sum giving total antero-posterior dimen- 

 sion), length, D, of shells, with a modification of the stadiometer used 

 by Davenport in his studies on "Evolution of Pecten" (13).* 



* The stadiometer as made by the writer consisted of a nearly circular box-like 

 base of sheet-iron about fourteen inches in diameter and one-half inch in width ^ 

 one end of which was squared. To this end in the cavity of the box was soldered 

 in an inverted position the upper jaw of a vernier caliper like that already men- 

 tioned, so that the zero-point coincided with the upper surface of the box. The 

 upper jaw of the caliper was free to move up and down the graduated scale. On 

 the upper surface of the box there was then pasted metrically ruled paper, and a 

 zero-point established to coincide with the zero-point of the caliper, after which 

 the centimeter spaces were numbered to the right, to the left, and to the opposite 

 end of the instrument. In using the instrument it was the custom to take the 

 most convenient valve of the mussel, place it upon the base of the instrument with 

 the umbo, or its probable location, as far as could be determined, on the zero-point. 

 Dimensions AD and PD could be readily taken, and if desired, DVD, and by 

 manipulation of the free arm of the caliper until flush with the surface of the 

 valve, one-half of the dextro-sinistral diameter from the graduated arm of the 

 caliper. 



