144 Miscellaneous. 
sonian Institution embrace specimens of Thalassidroma Leachii 
killed about Washington in August 1842, with hundreds of others. 
I myself obtained at Harrisburgh, Penn., a fine adult Pomarine 
Skua (Cataractes pomarinus), killed on the Susquehanna, near that 
city, in September 1839. Adults of the species mentioned are 
rarely seen within the limits of the United States at all, and in sum- 
mer the latter would hardly be likely to occur south of Newfoundland. 
The present is not the occasion to discuss the nature of that im- 
pulse which causes the bird or the fish to retrace its steps in spring 
so unerringly ; the fact is a well-established one, and of much im- 
portance in reference to the multiplication or diminution of species. 
A region deprived of its spring birds or fishes by extermination will 
only be filled up again in the course of a long period of time. The 
result, however, can be greatly accelerated by artificial propagation 
in the places to be supplied. 
It may be considered established that the migrations of birds 
are generally more or less in a north and south direction, influenced 
very materially by river-courses, mountain-chains, forests, conditions 
of moisture, mean temperature, altitude, &c. Middendorf (Die 
Isepiptesen Russlands) suggests that birds migrate in the direction 
of the magnetic pole—a suggestion not at all borne out by the facts 
in North America. 
It may be further remarked that while birds proceed generally in 
the spring to the very spot of birth, and by a definite route, their 
return in autumn is not necessarily in the same line. Many birds 
are familiar visitors in abundance, in certain localities, in either 
spring or autumn, and are not known there in the other season. 
This is a fact well known to the diligent collector; and I have been 
inclined to think that, in very many instances, birds proceed north- 
ward along the valley of the Mississippi, to return along the coast of 
the Atlantic. 
In general the northward vernal movement is performed much 
more rapidly, and with fewer stops by the way, than the autumnal. 
Birds generally make their appearance in given localities with 
wonderful regularity in the spring, the Sylvicolide especially—a 
difference of a few days in successive years attracting the notice of 
the careful observer; this difference is generally influenced by the 
season. The time of autumnal return is perhaps less definite. 
Observations on the Microscopic Shell-structure of Spirifer cuspi- 
datus, Sow., and some similar Forms. By F. B. Merk. 
Mr. Meek shows in a paper in Proc. Acad. Nat. Se. Philad. 1865, 
p. 275, that the shell of the Spirifer cuspidatus, both of American 
specimens referred to this species, or closely related, and of an Irish 
specimen of this species received from Mr. Davidson, is clearly punc- 
tate, contrary to the decision of Dr. Carpenter. He then asks the 
question whether two types—a punctate having the internal charac- 
ters of Syringothyris, and an impunctate—may not be included under 
the species, and suggests the importance of observations with refer- 
ence to this question.—Silliman’s American Journal, May 1866. 
