136 



Monograph of the Crustacea of the Cincinnati Grovp. 



m 



to ten segments, havnig large facets, and distinct, wide, nearly straight 

 pleural grooves, not reaching the margin ; glabella indistinctly defined 

 in front ; pygidium, with the segments, usually indistinctly marked, 

 the axis generally distinct and annulated, Avhen traceable always 

 elongate conic (of the ordinary type). 



Contains the following subgenera : 1st, Asaphtis; 2d, Isotelus. 



Tracks of an Asaphus (?) 



This figure ought, upon its face, to carry conviction that it represents 

 the tracks made by the feet of some animal. There 

 are, however, some additional facts that may be here 

 stated. On the same slab, about two inches to the left 

 hand of these tracks, there are four similar impressions, 

 that seem possible to have been those of the left feet, if 

 the figure shows the tracks of the right feet. On an- 

 other slab there are as many tracks as those here en- 

 graved, which follow each other in the same order, as 

 i well as a number of tracks Avhich indicate movements 

 in other directions. I think it quite clear that there 

 are too many marks of the same kind, which are regular 

 in their order, to leave any reasonable doubt about their 

 origin. 



They are presumed to be the tracks of a crustacean, 

 because no animal of any other class known to the Silu- 

 rian rocks could have made them. It being admitted 

 that they are the tracks of a crustacean, the conclusion is 

 inevitable that they are either the tracks of an Asai^hus 

 or of an unknown animal. If an Asaphus made tracks, 

 and it is the opinion of naturalists that it did, or might 

 have done so, they would have been as large as these, 

 and, for ought that is now known, these are just such 

 tracks as it would' have made. While such is the proba- 

 bility, we would not, of course, resort to an hypothesis 

 based upon an unknown fossil. 



The foot seems to have been trifid more than two thirds 

 of its length, and if webbed, as the impression indicates, 

 would have been a good paddle for swimming. The 

 posterior part of the foot, instead of leaving an impres- 

 sion, has left an elevation. This is an anomaly not easily 

 solved, unless upon the hypothesis that the web feet were 

 ">" used generally for swimming, and not for walking, and 



^'iuA^phlf^f that when used for walking, the folded web, at the pos- 



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