1883.] Zeblogy. 1297 
Its distribution hitherto, it will be seen, is Europe and South 
America; inhabiting fresh waters in mountainous regions, and it 
is interesting to notice that all the species of the genus as yet de- 
scribed occur in mountainous districts, an instance of correlation 
of form to external conditions. 
Pyramid lake, although it receives the fresh water of the 
Truckee river, the outlet of that gem of lakes, Tahoo, is very 
strongly alkaline, and the water is not good for human use, al- 
though it can be used fora short period without much incon- 
venience, 
The elevation of Pyramid lake is 4890 feet, and Walker’s lake 
has an altitude of 3840 feet; the water is brackish. 
These lakes are the remnants of the great Tertiary lake which 
covered this general region, and are the pockets or deeper depres- 
sions in the floor of the ancient lake. 
Pyrgula nevadensis is a small shell, of five to six whorls, which 
are traversed spirally by a single strong keel or carina. It is 
white, smooth and glossy, and measures eighteen-hundredths of 
an inch in length by about half as much in breadth. It occurs 
also in a calcareous deposit with Pompholyx, another curious form, 
as well as in the dredgings. 
SEXUAL CHARACTERS OF LimuLus.—It has often been puzzling 
to account for the fact that no cast-off shells of the Limulus, 
young female are very abundant. During the past five years I 
few 
fe incorrect, as the living and dead males are often found, per- 
aps quite as often as the grown females. | ci 
uring the past sence in our shore trips from 2 T 
Commission laboratory at Woods Holl, Mass., I collect "i rg 
_ umber of cast-off shells of the Limulus, also a few small one 
- Specimens, none of which possessed the modified claw © i 
ale. (F ig. 1 B shows the well-known claw of the male, an 
a Li .) ital y 
. Upon further examination it was found that the geni T 
'Ngs, located on the under side of first pair of abdominal appen: 
