136 ORIGIN, ETC., OF THE FAUNA. 
apparently in tropical Asia. The most plausible explanation of these facts is that 
Orthoporus and its near allies were evolved in a continent embracing and connecting 
South America and tropical Africa, and that Orthoporus moved northwards into 
Central America after the formation of the isthmus of Panama. In Central America 
the genus ranges from Panama to Mexico. 
Like Orthoporus, the genus Epinannolene also appears to be South American in its 
affinities. 
Although the precise geographical range of the two dominant Central American 
genera of Spiroboloidea, namely Rhinocricus and Spirobolus, has yet to be ascertained, 
one or two facts of interest may be noticed. 
Spirobolus, ranging from Mexico to Guatemala, occurs also in the Southern States 
of North America and in Eastern Asia as far north as Pekin. It is not abundant in 
the West Indies, although recorded from Porto Rico. It is apparently absent from 
South America. The evidence, therefore, is in favour of a northern, rather than a 
southern, origin for this genus. | 
Rhinocricus, on the other hand, is abundant in South America and the West Indies, 
but not in North America. In Central America it extends from Panama to Mexico. 
It is also the dominant genus of the family Spirobolide in the Oriental Region, but 
does not appear to extend in Eastern Asia so far north as Spirodolus. Clearly, there- 
fore, it is a more southern type than the latter. 
There is at present no agreement amongst systematists as to the number of families 
into which the Polydesmoidea should be divided, but, setting aside some of the peculiar 
forms referred to below, sufficient is known of the inter-relationship of some of the 
other genera to make instructive comparison between the Polydesmoid fauna of 
Central America and that of other countries. 
The most important of these belong to the families Platyrhachide and Chelo- 
desmide. 
The two subfamilies of Platyrhachide are represented in the tropical south-eastern 
countries of Asia, the Platyrhachine being especially abundant in Malaysia. They are 
not represented in tropical Africa. In America they inhabit the northern countries of 
South America, particularly Colombia and Ecuador, whence the Platyrhachine extend 
northwards, at all events, to Costa Rica, and occur in the Antilles; whereas the 
Euryurine have a much more northern extension, passing from Costa Rica, through 
Guatemala and Mexico, into the Southern States of North America. 
The Chelodesmide are very wide-ranging, but from our present point of view the 
most interesting forms are the species of the dominant Central American genus 
Rhysodesmus of the subfamily Xystodesmine. This genus is represented by a vast 
number of species in the northern districts of Central America, but is apparently 
absent in South America and in the Antilles. It extends into North America, and 
closely allied, if not generically identical, forms occur also in China, 
