PACKARD. — PALEOZOIC ARTHROPOD TRAILS. 71 



While these tracks are certainly not Isopod tracks, they with still more 

 certainty cannot be referred to impressions made by the feet of insects, 

 which are always alternate, as in hexapod insects the legs of each pair 

 arc raised and put down alternately. The Providence carboniferous 

 tracks were evidently made by an arthropod of the same group as 

 Limulus, as the tracks are opposite, and in general shape like those of 

 Limulus, as may be seen by Fig. 1. The present tracks differ from 

 those of Limulus in the absence of a caudal spine trail-mark, and in the 

 fact that an additional anterior pair of feet made impressions. Trilo- 

 bites are not known to exist in our Upper Carboniferous associated with 

 Limulus. Lirnuloids of the genus Prestwichia with a short caudal spine 

 exist in considerable numbers in the Upper Carboniferous of Mason 

 Creek, Illinois, and in the Upper Carboniferous beds of Pennsylvania, 

 though from a horizon higher than that of the Mason Creek beds. As 

 the genus Belinurus has a very long caudal spine, and there are uo traces 

 of a median furrow in our trails, that genus should be ruled out as the 

 author of these tracks. Now the adult Prestwichia is about two inches 

 in diameter and its caudal spine nearly half an inch in length. It is, 

 however, well known that in the freshly hatched Limulus, and even after 

 the first moult, and when the creatures are half an inch in diameter, the 

 caudal spine is very short, too much so, probably, to leave a trail or 

 median furrow. 



The Providence trails are considerably less than half an inch in diam- 

 eter, and reasoning by analogy, and also in part by exclusion, it seems 

 not impossible that these trails are the footprints of a young Prestwichia 

 a little less than half an inch in diameter, with too short a caudal spine 

 to leave a furrow. 



This conclusion is interesting as suggesting the occurrence of these 

 Lirnuloids in the Narragansett basin during the later part of the Carbon- 

 iferous Period. These tracks are so similar to those of the Chemung beds 

 above described that they were probably made by Merostomes of the 

 same family or genus, and may be referred to Merostomichnites rather 

 than to Protichnites. 



The possibility that these trails could have been made by an Eurypterid 

 seems excluded by the absence of a median furrow, or of any prints made 

 by the large paddles of the hind feet, or by the paddles and chelae of the 

 first pair of feet of such a form as Pterygotus. 



Brown University. 



