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AGASSIZ AND MAYER: ACALEPHS FROM THE FIJI ISLANDS. 159 
Fishes, Crustaceans, Mollusks, Worms, Echinoderms, and Polyps dredged 
off the west coast of Central America there were familiar West Indian 
types, or east coast forms. 
A recent survey of the region of the Isthmus of Panama has been 
made by Hill,t who considers that it is impossible to make any serious 
deductions concerning the relations of North and South America during 
epochs preceding the Jurassic period, owing to the lack of data. He 
also concludes (p. 261) that the waters of the Atlantic and Pacific were 
probably as completely separated by a great continental land barrier in 
Cretaceous times as they are to-day, a proposition fully as tenable as the 
opposite hypothesis that they were united. If the marine passage ever 
existed across the I“thmus of Panama, or elsewhere in Tropical America, 
it must have been during the later Eocene period, and this strait was 
probably of a shallow and restricted character, and had finally disap- 
peared before the close of the Miocene period. 
If straits of considerable width and depth have ever connected the 
waters of the Tropical Atlantic with those of the Pacific, it is probable 
that the great Equatorial current would pour through them from the 
Atlantic into the Pacific, and thus the Pacific Ocean would become im- 
pregnated with Atlantic species. Once having gained access to the 
Pacific, the westerly Equatorial set would soon distribute the pelagic 
animals widely over the ocean, 
It is interesting to notice that while so many characteristic types of 
Tropical Atlantic Medusæ are also found in the Pacific, the most emi- 
nently characteristic Tropical Pacific genera, the Rhizostome have re- 
n the Atlantic.? This, indeed, is what we 
markably few analogues 
should expect as a result of the westerly set of the great Equatorial 
enrrent that would freely sweep animals from the Atlantic into the 
Pacific, but would in like measure hinder the entrance of Pacific forms 
into the Atlantic. 
Of the thirty-eight species of Acalephs found by us in the Fiji Islands, 
twenty-six are new to science, 
1 Hill, R. T., 1898, The Geological History of the Isthmus of Panama, etc. 
3ull. Mus. Comp. Zovl., Geological Series, Vol. NN VIII. No. 5, pp. 151-285, 
Plates L-XIX. 
2 In this connection, see Lendenfeld, R. von, 1884, The Geographical Distribu- 
tion of Australian Seyphomedusæ, Proc. Linn. Soc, New South Wales, Volv Ta. 
pp. 421-433. Also, Vanhoffen, E., 1888, Bibliotheca Zool., Bd. I. Heft 3, p. 46, 
Map. 
