250 S. W. Ford—Additional Embryonic Forms of Trilobites. 
than in low latitudes. In passing from the southern to the 
northern boundary, if we compare localities of equal altitudes 
along any given meridian, we shall find the rainfall steadily 
though perhaps not uniformly increasing. This is an obvious 
consequence of the theory suggested. re 
Although no very great effects upon the general condition 
of aridity are here attributed to the depletion of moisture by 
the passage of the winds over mountain ranges, it is stil] true, 
no doubt, that highly important local effects are thereby pro- 
uced. The rainfall at the eastern base of the Sierra Nevada, 
and for two hundred miles east of it is most probably reduced 
very greatly by this cause. In the sink of the Humboldt 
River, the annual precipitation seldom reaches four inches, and 
may average not more than three inches. But as we pass 
eastward beyond the wake of this range, its effects become 
gradually less; and long before the Wasatch is reached they 
have become inconsiderable. Since the Sierra Nevada is the 
longest, highest and widest of the individualized ranges of the 
Rocky system, its local effect upon the humidity of the plains 
and valleys lying immediately under its lee is greater than that 
of any other. But the same kind of effect is preceptible in 
some other ranges. : 
he discussion of the causes of local variations in climate 
might be almost indefinitely extended. Nothing more 1s 
designed here than to advert to one general cause of aridity 
which prevails over the entire region, and which everywhere 
persists, though it is often obscured, sometimes reversed and 
sometimes reinforced by local causes. 
Art. XXXVI.—On additional Embryonic Forms of Trilobites 
Jrom the Primordial Rocks of Troy, N. Y¥., with observatrons on 
the genera Olenellus, Paradoxides and Hydrocephalus; by - 
W. Forp. 
ner. As examples of the former we may instance Paradowides 
sptnosus from the Bohemian Primordial, and P. Bennetti from 
that of Newfoundland (and the majority of the Bohemian species 
