CONTRIBUTIONS TO PALAEONTOLOGY. 123 



greater thickness ; and these successive trilobite beds may be 

 representatives of that which, in other localities, will be found 

 characterized by a more considerable and distinct fauna. I make 

 this suggestion, from the facts which have fallen under my ob- 

 servation during investigations much too limited in time and area 

 to afford such satisfactory results as I could desire. 



It is an interesting and significant fact that the two most cha- 

 racteristic forms of DiKELOcEPHALus commenced their existence 

 near the close of the Potsdam period proper, passing into the 

 alternating lower beds of the Magnesian limestone ; while but one 

 other species of Trilobite has been found in the same association, 

 so far as my investigations have extended. 



It is perhaps premature to generalize from these few facts; but 

 according to our present knowledge of the Upper Mississippi 

 valley, we are compelled to regard the typical forms of Dikelo- 

 CEPHALus as characteristic of the close of the Potsdam period. 



In addition to the Trilobites, we have a single species of Agno- 

 STtts, and a new and remarkable Crustacean; the latter from the 

 higher beds of the sandstone, or within about fifty feet of the 

 Lower Magnesian limestone. • 



The fauna of this sandstone, other than that of the crustaceans, 

 is comparatively meagre, consisting of a few species of Lingula, 

 a DisciNA, an Obolella?, an Orthis, two Gasteropods, one Theca 

 and a Serpulites ? 



Of the lower orders of animal life, we have very little at our 

 disposal for its illustration. We know, however, that the family 

 of GRApTOLiTiDiE appeared in beds which are somewhat above 

 the centre of the formation, Crinoidal columns are cited by Dr. 

 Owen as occurring in several localities, and I have seen a single 

 obscure specimen which is apparently referable to this family of 

 fossils. 



The absence of animal life, in greater variety of forms, seems 

 to be due to the unfavorable character of the sediment, rather 

 than to any other cause. From the fact that every bed, however 

 thin, where there is an admixture of sand with argillaceous mat- 

 ter, or with a small portion of calcareous matter, is seized upon 

 as it were for the habitations of these animals, it is evident that 

 it needed only a more favorable condition of the ocean bed to 

 have developed an extensive fauna. We are, I think, moreover 

 warranted in concluding that at some not far distant but now 

 unknown point, this condition did exist in a much higher degree 

 than in the localities investigated. 



