BRACHIOPODA—THOMSON. 39 
1,000 fathoms) and a gradual migration across the deep oceans along the bottom is 
impossible for other species. The majority of the Brachiopods are found on the 
submarine slopes of the continents and the neighbouring islands, and the deep oceans 
are barriers which they cannot cross. 
Cases of discontinuous distribution of the shallower water forms have therefore 
a profound significance. Thus the occurrence of Dyscolia wyvillei, Lacazella medi- 
terranea, Platidia anomioides, and Hucalathis ergastica in the Antilles and on the 
corresponding coasts east of the Atlantic, together with the occurrence of other closely 
allied species on these opposite coasts and of two Antillean species on Ascension Island, 
can have only one explanation, viz., the occurrence of land connections or at least 
shallow submarine ridges across the Atlantic in an earlier geological period. 
In the case of the discontinuous distribution of the deep water species Dyscolia 
wyviller in the north Indian Ocean and east and west of the Atlantic, together with 
the restriction of Chlidonophora to these two regions, Blochmann seeks a similar 
geological explanation, viz., that in Tertiary times there was an open seaway between 
the Atlantic and the Indian Ocean through the earlier greatly extended Mediterranean 
Sea. The occurrence of Kingena alcocki Joubin in the Indian Ocean is similarly 
correlated with the fossil occurrence of this genus in Europe. 
Before these conclusions can be regarded as definitely established, Blochmann 
points out that more must be learnt of the structure of the larvee and of the duration 
of the free-swimming period in a larger and more varied series of genera. Meanwhile 
they possess a high degree of probability, and they suggest that a study of the 
distribution of southern Brachiopods may be a useful contribution to the larger problem 
of the former land connections of the southern hemisphere. 
Schuchert (1911), in discussing the paleographic and geological significance of 
recent Brachiopoda, has analysed and discussed the bathymetric and geographical 
distribution of the recent genera. The districts recognised are a deep-water realm 
and four shallow-water geographical regions as follows :—Boreal, Austral, Oceanica, 
and Gondwana. The analysis contains several minor inaccuracies, but these do not 
greatly affect the main conclusions. A more serious defect is the apparent assumption 
that the origin of nearly all the southern forms must be sought in the north, but that 
none of the northern forms originated in the south. Yet the southern Tertiary faunas 
were at least as rich and varied as those of the north, and there is little reason to assume 
that such was not also the case in earlier times. 
Schuchert follows Blochmann in postulating a former land connection across the 
Atlantic, the northern shore of ancient Gondwana and the southern strand of the 
extensive Mediterranean Tethys which reached from Central America to India. This 
shore was broken up in the present south Atlantic region during the early Eocene, but 
in the early Tertiary there was an open seaway between the Panama and Caribbean 
regions and also free communication with the Indian Ocean. The genera of the northern 
