124 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



The oldest chilopods are species described by Scudder (Mem. Boston Soc. Nat. Hist., 

 vol. 4, 1890, p. 417, pi. 38) from the Pennsylvania!! at Mazon Creek, Grundy County, Illinois. 

 Only one of these, Latzclia primordialis Scudder (pi. 38 fig. 3), is at all well preserved. 

 This little animal, less than an inch long, had a depressed body, with a median carina, exceed- 

 ingly long slender legs, and about nineteen segments. The head is very nearly obliterated. 



DIPLOPODA. 



The diplopods, especially the polydesmids with their lateral outgrowths, often have a 

 general appearance somewhat like that of a trilobite, but on closer examination few like- 

 nesses are seen. The most striking single feature of the group, the possession by each seg- 

 ment of two pairs of appendages, is not in any way foreshadowed in the trilobites, none 

 of which shows any tendency toward a fusion of pairs of adjacent segments. The anten- 

 nules are short, antennae absent, mandibles and maxilluke much modified, the latter possibly 

 biramous, and the maxillze absent. The trunk appendages are very similar to those of chi- 

 lopods, and could readily be derived from the endopodites of trilobites. 



The oldest diplopods are found in the Silurian (Ludlow) and Devonian (Lower Old 

 Red) of Scotland, and three species belonging to two genera are known. The oldest is 

 Archidesmus loganensis Peach (1889, p. 123, pi. 4, fig. 4), and the Devonian species are 

 Archidesmus macnicoli Peach and Kampecaris forfarensis Page (Peach 1882, p. 182, pi. 2, 

 fig. 2, 2a, and p. 179, pi. 2, figs. i-ig). All of these species show lateral expansions like 

 the recent Polydesmidae, and these of course suggest the pleural lobes of trilobites. All 

 three of the species are simpler than any modern diplopod, for there is only a single pair 

 of appendages on each segment. No foramina rcpugnatoria were observed, and the eyes of 

 Kampecaris forfarensis as described are singularly like those of a phacopid. 



Peach says: "The eye itself is made up of numerous facets which are arranged in 

 oblique rows, the posterior end of each row being inclined downwards and outwards, the 

 facets being so numerous and so close together that the eye simulates a compound one." There 

 is also a protecting ridge which somewhat resembles a palpebral lobe (1882, pi. 7, fig. la). 

 Peach comments on the strength of the test, and from his description it appears that it must 

 have been preserved in the same manner as the test of trilobites. It was punctate, and gran- 

 ules and spines were also present. The presence of the lateral outgrowths in these ancient 

 specimens would seem to indicate that they are primitive features, and may have been in- 

 herited. While possibly not homologous with the pleural extensions of trilobites, they may 

 be vestiges of these structures. 



The limbs are made up of seven segments which are circular in section and expand at 

 the distal end. The distal one bears one or two minute spines. They are most readily com- 

 pared with the endopodites of Isotclus. The resemblance is, in fact, rather close. The 

 sternal plates are wider and the limbs of opposite sides further apart than in modern diplo- 

 pods. Except for one pair of antenna;, no cephalic appendages are preserved. 



While these specimens do not serve to connect the Diplopoda with the Trilobita, they 

 do show that most of the specializations of the former originated since Lower Devonian 

 times, and lead one to suspect that the derivation from marine ancestors took place very early, 

 perhaps in the Cambrian. If no very close connection with the trilobites is indicated, there 

 is also nothing to show that the diplopods could not have been derived from that group. 



