ARCTIC MOLLUSCAN FAUNA, 489 
for I have not unfrequently done the same; yet | am satisfied that 
neither he or any other person could have’ collected more care- 
fully or more successfully than was done at the many different 
localities visited by members of the aes app in Grinnell and 
Hall’s Land. 
Having now stated my facts, I beg to advance the opinion that 
the disproportion in the number of individuals in the species of 
Conchifera and Gastropoda must be due to the presence or absence 
of suitable food; thus the four species of Conchifera mentioned as 
occurring, both recent and fossil, in great numbers, subsist on 
diatoms, and the abundance of those forms in the Polar water gives 
an easy explanation for the presence of these Mollusca. A supply 
of suitable food is evidently not forthcoming for the support of the 
Siphonostomata; consequently the limited number of individuals 
in that order. 
A consideration of the ocean temperatures and tidal movements 
of Smith Sound and northwards is, in-itself, a sufficient reason 
why its molluscan fauna should change very materially from that 
now existing south in Davis Strait. Putting in the back ground 
the distance of 1200 miles of latitude, we find that the Davis Strait 
tide is met at Cape Frazer by the icy cold water of the Paleucrystic 
Sea escaping down Robeson Channel. In all probability this frigid 
stream occupies the entire bottom of Smith Sound; but its tem- 
perature becomes somewhat modified as it debouches into Baffin 
Bay, and is overspread by the warmer waters of the Davis Strait 
tide. 
From whence, then, are we to consider the molluscan fauna of 
Smith Sound has been derived? If from Davis Strait, then we 
must contemplate a movement of species from the comparatively 
warm area indicated by a bottom temperature of 34° to 36°,* 
meeting and invading a strong southerly setting glacial current of 
29°, the normal temperature of the water in the Polar Ocean. 
Is it not natural that, under such circumstances, a great number 
of Davis Strait species should have failed to enter Smith Sound? 
It is, on the other hand, possible that the entire molluscan fauna 
of Smith Sound derived its origin from the Polar Basin, and is not 
indebted at all to any introduction of molluscan forms from Davis 
Strait. 
Incomplete as is our knowledge of the molluscan fauna of the 
* Cruise of ‘ Valorous,’ Proc. Roy. Soc. 1876. 
