Page:The Zoologist, 3rd series, vol 1 (1877).djvu/465

Rh for I have not unfrequently done the same; yet I am satisfied that neither he or any other person could have collected more carefully or more successfully than was done at the many different localities visited by members of the Expedition 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 Palæocrystic Sea escaping down Robeson Channel. In all probability this frigid stream occupies the entire bottom of Smith Sound; but its temperature 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