582 ARTHROPODA. 



poda), on the other hand, in which the segments are amalgamated 

 in pairs, the stigmata are placed on every apparent ring. 



The recent Myriopods are grouped into the four orders of the 



Chilopoda (Centipedes), Diplopoda (Millepedes), Pauropoda (Pauro- 



pus), and Onychophora (Peripatus). To these Scudder has added 



the two extinct orders of the Protosyngnatha and Archipolypoda for 



the reception of the Palaeozoic types of Myriopods. 



As regards their distribution in time, all the recent Myriopods 

 are terrestrial in habit. As a necessary result of this, the remains 

 of Myriopods are not abundant as fossils, and have been mostly 

 found in deposits of distinctly fresh-water or estuarine origin. The 

 occurrence of Myriopods in unequivocally marine strata is, however, 

 by no means unknown, though difficult with our present knowledge 

 to satisfactorily explain. The two living orders of the Pauropoda 

 and Onychophora — represented each by a single genus only — are 

 unknown as fossils, and require no further consideration here. The 

 other two existing orders — viz., the Chilopoda and Diplopoda — are 

 represented only by Tertiary types, with the possible exception of a 

 Cretaceous form of the latter order. On the other hand, a con- 

 siderable number of Palaeozoic Myriopods are known, which differ 

 more or less widely from all existing types, and for the reception of 

 which Mr Scudder has founded the two orders of the Protosyngnatha 

 and Archipolypoda. The only known representative of the Proto- 

 syngnatha is the genus Palceocampa, which is found in the Coal- 

 measures of North America. The oldest Myriopods, however, 

 belong to the Archipolypoda, and remains of early forms of this order 

 have been described by Mr Peach as occurring in the Old Red 

 Sandstone of Scotland. It is, however, in the Carboniferous rocks 

 that this ancient group of Myriopods attains its maximum develop- 

 ment, about thirty different species having been already described 

 from the Coal-measures of the Old and New Worlds. There are 

 also a few Myriopods known to occur in the Permian rocks of 

 Europe, which probably belong to the same order. In the follow- 

 ing a brief account is given of the four orders of Myriopods which 

 are known to be represented by fossil forms. 



Order I. Protosyngnatha. — This order comprises only the 

 single genus Palceocampa, of which only a single species (P. 

 anthrax) has been hitherto recorded. In this remarkable genus 

 (fig. 438) the body is comparatively short and worm-like, consisting 

 of few segments, each body-ring being furnished with a single dorsal 

 and ventral plate. Each segment carries a pair of stout fleshy legs, 

 and the upper surface is furnished with large tubercles, each of 

 which supports a cluster of long needles, and which are arranged in 

 longitudinal rows. The sole known species of Palceocampa is found 

 in the Coal-measures of Illinois ; and the possession of the bundles 



