PRESIDENTS ADDRESS—SECTION C. 67 
years, by the investigations of Mr. T. 8. Hall, the number of 
identities has been reduced, whilst other endemic species have 
been established. 
IX. Camprian.—About forty species in all classes have been 
recorded from the few known localities yielding fossils of Lower 
Cambrian age; of these only two have received names borne by 
American species, namely, the molluscs Hyolithes micans and 
Stenotheca rugosa. For these identifications I am reponsible, 
but as they are based on figures and descriptions, and not by 
actual comparisons, they are open to revision, or, if correct, it 
is not improbable that those molluscs were of pelagic habit. 
In conclusion, life at the several epochs is not common be- 
tween Australia and Western Europe or North America, though 
in the present state of our knowledge a few cosmopolitan species 
appear at some of the epochs; in general the older the age, the 
greater the number of such, and the greater the similarity be- 
tween the contemporaneous faunas. 
II. Is Migration oF Species PosststE Across WIDE AND ABYSSAL 
Oceanic AREAS? 
The bathymetrical distribution of mollusca in the North 
Atlantic proves that the greater bulk of the species is limited 
to the shallow slopes bounding the deep plateau, and that only a 
very limited number extends into abyssal depths ; the forms in- 
habiting these regions are of low organisation, of very few 
species, and of widely extended distribution. 
The presence of some coastal species on either side of the 
North Atlantic does not imply migration across the intervening 
oceanic expanse, else they should appear in the intermediate 
oceanic floor, which they do not. 
On the other hand, they may have crossed while in the free- 
swimming, or “ veliger stage.” I cannot obtain definite infor- 
mation as to the duration of the free swimming stage of a 
mollusc, though the sum of the evidence implies days only, not 
extending to weeks, so that, even aided by oceanic drifts, it is 
impossible for a living embryo to be carried from one land 
margin to the other on either side of the North Atlantic. 
Macgillivrayia pelagica, a muricoid embryo, was taken 15 miles 
from shore. Cooke, in ‘‘ Molluses,” page 146, says of the larve of 
Dreissensia, “they pass about eight days on the surface,” and 
though the Glochidium stage of Unio cannot swim, yet in about 
four weeks after—the Glochidium has quitted its host. 
The occurrence of identical species on opposite sides of the 
North Atlantic must, therefore, be explained on other migratory 
routes, though it is possible that a mode of dissemination of 
certain species may be by the agency of driftwood or floating 
seaweed. 
E 2 
