36 PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 125 
they depend, or through a combination of both, is difficult to say 
without experimental evidence. 
GEOGRAPHICAL DISTRIBUTION.—On the basis of their recorded 
distributions (including the localities herein reported), the species 
represented in the buoy collection may be grouped geographically as 
follows. 
Five isopod and one tanaidacean species (about 20 percent of the 
total) are widely distributed, occurring at least in both the Atlantic 
and Pacific oceans: Cirolana parva, Idotea balthica, I. metallica, 
Leptochelia dubia, Sphaeroma terebrans, and S. walkeri. It should be 
noted, however, that the distribution of this group in the Pacific is 
rather skewed. Only one of these (Leptochelia dubia) has been reported 
from the eastern Pacific, three of them (L. dubia, Cirolana parva, 
and Sphaeroma walkeri) are now known from the central Pacific, but 
all six occur in the Indo-Pacific region. Interestingly, Leptochelia 
dubia is the only species in the collection known to occur on both the 
Atlantic and Pacific coasts of the United States, as well as in the 
Bahamas and Hawaiian Islands. Two other cosmopolitan species taken 
in Hawaii, Cirolana parva and Sphaeroma walkeri, also occur off the 
Florida coast but not on the Pacific coast. Present knowledge of the 
isopod and tanaidacean faunas of the vast Pacific is much too incom- 
plete as yet for safe generalization. 
The remaining 80 percent (approximately) of the species are 
separated by the continental barrier—nine on the eastern side and 
14 on the west coast (fig. 2; table 2). In at least one case, an eastern 
and western species comprise a “species pair,” that is, morphologically 
closely related species occurring under similar ecological conditions on 
opposite sides of a major barrier. Fitting this definition are Para- 
cerceis caudata found on buoys in warm waters of the Atlantic, Carib- 
bean, and Gulf of Mexico, and its close morphological relative, P. 
sculpta, taken from buoys in tropical-subtropical waters in southern 
California and Hawaii (fig. 3). The Atlantic and Pacific coast species 
of Idotea also may be ecological equivalents in temperate waters. 
On both sides of the continent, the species may be divided roughly 
into northern and southern groups, although a few species have 
extended north-south ranges along the coast. The locality data are 
too limited for precise definition of boundaries between northern and 
southern faunas and certainly for any subdivision of these groups. 
For sake of analysis, however, two well-known temperature and biotic 
break-points were selected—Cape Hatteras, at 35%°N latitude on 
the Atlantic coast and Point Conception at 34°N latitude on the 
Pacific coast (Hutchins, 1947). 
Although the two break-points are nearly at the same latitude, the 
temperature changes they signify and the temperature zones they 
