On Ascension-Island Zoology. 431 
Agulhas current, which issues from the Indian Ocean and 
flows round the Cape of Good Hope northwards towards 
St. Helena, and thence past Ascension to the West Indies ; 
and to the Guinea current, as well as to a passage which 
formerly existed across Africa in the line of the Sahara, may 
be owing the partial correspondence between the Mollusca 
of the Indian Ocean and of the Mediterranean. Yet, if these 
ocean currents be the cause of such distribution (and doubtless 
they must influence it considerably), we should expect to find 
these same species in the Indian Ocean or at the Cape. We 
do not; and are we therefore to consider St. Helena the 
starting-point of these species? Unfortunately the Hipponyx 
is found on the west side of both North and South America, 
afact eminently perplexing; and Purpura hemastoma has yet 
to be proved distinct from P. biserialis of the Californian and 
Panama coasts. 
Purpura ascensionis appears to be peculiar to the island, 
and, although living under the same conditions as the other 
species, has not been similarly affected by these oceanic cur- 
rents, unless the P. neritoidea found at the Cape-Verd Islands 
and Benguela (Dunker) is to be considered a modified form 
of it, or vice versa. 
Nerita ascenstonis 1s also limited in range; and the only 
other locality known to me whence it has been obtained is 
the island ot Trimidad, off the Brazilian coast, about 20° 
south of the equator. In this instance also to the ocean cur- 
rents may be attributable such distribution ; for, according to 
certain maps which I have consulted, a branch of the great 
Indian-Ocean current passing Ascension sweeps southward 
along the Brazilian coast past the island of Trinidad. But 
whether such speculations respecting the range of these mol- 
lusks possess any real value is very questionable, seeing how 
extremely imperfect up to the present time has been the in- 
vestigation of both the east and west shores of the South 
Atlantic or even of the whole of the Caribbean Islands. 
The species of Malleus is very closely represented in the 
West Indies by M. candeana of D’Orbigny, which, however, 
may be specifically distinct. The only land-snail, Helix simi- 
laris, is, with regard to distribution, quite an anomaly among 
terrestrial gastropods, being met with in Brazil, Cuba, Natal, 
Mauritius, Madagascar, Seychelles, Rodriguez, Bourbon, 
Bengal, China, Java, Philippines, Australia, Sandwich 
Islands, and other places. Monsieur Morelet thinks that it 
probably originated in the eastern parts of Asia, whence it 
has spread on the one hand to Polynesia, and on the other to 
America. 
