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Show 346 MR. R. I. POCOCK OX THE [Apr. 21, probably be looked to for its ancestry. There seems no reason to doubt that it originated in the area it now occupies. Families A t y p id ^:, B r a c h y b o th r i id .e , M ecicobothriidjE. The genera of Atypida?, two in number, scarcely pass south of the Equator. Atypus, occurring in the Mediterranean Region, and spreading northwards into Central Europe, beyond the 50th parallel of north latitude, is the most northern type of the Mygalo-morphse. It is also met with in Japan, Burma and Java, and in North America, where it ranges to the east of the Mississippi from Wisconsin (45th parallel of latitude) to Florida. Calommata is more restricted and more southern in range. It is confined to the Old World, and has been recorded from Japan, Siam, Burma, Sumatra, Java, and the Camaroon area of tropical West Africa. The Brachybothriidse contain the genera Acattyma from Japan, and Brachybothrium and Atypoides from North America, the latter from California, the former from British Columbia, North Carolina, Texas, &c. The Mecicobothriidse comprise two genera-Hexura from the two north-western States of North America (Washington and Oregon); and Mecicobothrium from the Argentine. Atypus seems to be a genus which, like Pachylomerus and others, extended in Tertiary times sufficiently far to the north to avail itself of the land-connection that then existed across the area now covered by the Behring Sea. Its disappearance from the countries to the north of its present distributional area must be assigned to the refrigeration of this region of the globe with the advent of glacial conditions. The distribution of Calom/mata in the eastern part of the Oriental Region and in Tropical West Africa has many parallels amongst the mammalia, and points to the former extension of the genus across an intervening forest-clad tract, and its survival in districts where the conditions remained favourable to its existence. The explanation given of the distribution of Atypus applies equally to the Brachybothriidse, except that Atypoides has no representative in the Old World. Brachybothrium extends in Western America northwards to Queen Charlotte Island, that is to say to the latitude of the Aleutian Islands and the Alaska Peninsula. Hence its similarity to, perhaps identity with, the Japanese Acattyma is no matter for surprise. The apparently discontinuous distribution of the Mecico-bothriida? is very remarkable ; but since the only known example of Mecicobothrium measures only 6 mm. in total length, it is highly probable that the existence of the genus in South America to the north of the Argentine has been overlooked. Considering the close structural similarity between Hexura and Mecicobothrium, and the admitted relationship between these two genera and the Brachybothriidpe and Atypidse, both northern groups, it seems |
