DIPLOPODA AND PROTOTRACHEATA, 137 
The Central-American Diplopods, from the faunistic standpoint, belong to three 
categories: (1) genera with northern affinities and probably derived from the north; 
(2) genera with southern affinities and probably derived from the south ; (8) 
restricted or almost restricted to the country and probably autochthonous. 
The principal groups of northern origin are the Platydesmide (Platydesmus), the 
Chordeumide (Cleidogona), the Luloidea (Paraiulus); Spirobolus of the group Spiro- 
boloidea and Rhysodesmus of the Xystodesmine Polydesmoidea. 
To the group with southern affinities and probably of southern origin must be 
referred the Spirostreptoidea (Orthoporus and Epinannolene), the Stemmiuloidea 
(Stemmiulus); Rhinocricus of the group Spiroboloidea and several of the Poly- 
desmoidea, like Ligiodesmus, Oncodesmus (Oniscodesmide), Lophodesmus (Pyrgo- 
desmide), Platyrhachus, Amplinus, and others (Platyrhachide). Of these, Orthoporus 
and probably Hpinannolene have African affinities; the Stemmiuloidea and Pyrgo- 
desmidee also have related genera in tropical Africa and Asia. On the other hand, the 
Oniscodesmidez and Platyrhachide are at present known only from South-Eastern Asia 
and tropical America. ‘The Oniscodesmide may await discovery in Africa, but the 
Platyrhachide appear to be certainly absent from that country. ‘Their restriction to 
Indo- and Austro-Malaysia and tropical America is singular. Nevertheless, since their 
distribution agrees in a measure, though not in exact detail, with that of the existing 
Tapiroid mammals, it may be that they are tropical survivors of families once extending 
throughout the countries now bordering the North Pacific Ocean. 
genera 
Of the peculiar, probably autochthonous, groups, the most interesting are the 
Polydesmoidea of the family Spheriodesmide, which appear to be a specialised offshoot 
of the Chelodesmide. The family ranges in Central America from Mexico to Costa 
Rica, and one of the genera, Cyclodesmus, is also represented in Jamaica. Also 
apparently restricted to Central America, with the exception of one form recorded from 
California, are the genera of the Chelodesmide belonging to the subfamily Rhacho- 
desmine. 
Class PROTOTRACHEATA. 
No fossil forms of this class are known. On morphological grounds it is believed 
to be of great antiquity, older even than the Trilobites, but, as Gadow has pertinently 
remarked :—* This genus [ Peripatus] need not date further back than the Cretaceous 
to allow us to account for the scattered distribution of its species” *. 
Until 1894 all the described species of this Class from South America, South Africa, 
and Australia, were referred to the single genus Peripatus, despite the ascertained 
existence of important morphological differences between those of the Continents in 
question. But since that date, when it was first proposed to give new generic names 
* Gadow here ignores the work of Bouvier and others, who refer the Prototracheata to several genera. 
BIOL. CENTR.-AMER., Introd. Vol., June 1915. T 
