Contributions to Palceontology. 175 



Fig. 15. A portion of a caudal spine of this animal. 

 Fig. 16. Another specimen of the same appendage. 



These remains occur in the upper portions of the sandstone ; and 

 thus far the specimens have been derived from two localities, one 

 near Miniska in Minnesota, and the other near Mazomania in Wis- 

 consin. 



This new and remarkable Crustacean is of great ,inte- 

 rest, since I believe no well-authenticated forms of this 

 class, other than Trilobites, have been found so low in the 

 series of formations. This fossil, moreover, if its relations 

 be such as I have supposed, is of great interest considered 

 in connexion with the Tracks in the Potsdam sandstone, 

 which occur both in Canada and Wisconsin. Whether 

 any relation may exist between the two, remains yet to be 

 proved by farther discoveries of specimens, and also of the 

 locomotive appendages which we may infer that this ani- 

 mal possessed, in character not entirely dissimilar to those 

 of Limulus. 



CONCLUSION. 



Although I have not been able to recognize the successive 

 Trilobite beds of the Sandstone as indicated by Dr. Owen, 

 I can nevertheless refer the species here described to three 

 different epochs in the Potsdam period ; and I am not pre- 

 pared at the present time to suggest any farther subdivision. 

 In the lower beds of the formation I have found Conoce- 

 phalites proper, together with Lingula, Lingulepis, Obo- 

 lella ? and Theca. In the middle stage, neither the limits 

 of the beds, nor the range of species or genera, have been 

 so well determined ; but grouping together all that I have 

 found between the well-defined upper beds and the lower 

 fossiliferous beds known, we have Conocephalites, Dike- 

 locephalus, Arionellus, Ptychaspis, Chariocephalus, 

 Ilmnurus and Agnostus, in the trilobitic fauna, together 

 with Orthis and Platyceras. The Graptolitil\e appa- 



