110 



Contributions to Palaeontology. 



" supposed to be the ventral, showing the muscular impressions." This 

 will afford the means of comparison with O.f polita, figures 20 & 

 21 on Plate 1. 



Obolella chromatica (Billings). 



The species of Obolella, cited by Mr. Billings from " the St. 

 " Croix river in the Western States/' and which he has identified 

 with Lingula? polita, has, according to that author, " the central 

 scars" "close together, one on each side of the median line and 

 " parallel." 



of such a form to the Falls of St. Croix: And furthermore I cannot find it 

 anywhere stated by Dr. Owen that the Lingula and Orbicula ( or any 

 Obolus or Obolus-like forms ) "from the Potsdam^ sandstone of the St. 

 "Croix river in the Western States," or the St. Croix river of Wisconsin 

 and Minnesota, ever occur associated with primordial Trilobites in that lo- 

 cality. Dr. Owen distinctly describes the "First Trilobite bed" as com- 

 posed of "Ferruginous Trilobite grits, Schistose sandstone containing 

 "forktailed Trilobite beds and Obolus layers, 1-8 feet ; Magnesio-calca- 

 "reous rock, with Obolus and forktailed Trilobite, 3 feet." The typical 

 locality of these beds is at Mountain island, and they are placed by Dr. Owen 

 above the "highly fossiliferous, schistose, silico-calcareous layers, interla- 

 " minated with argillaceous marly beds charged with sulphate [sulpku- 

 "ret] of iron; the former full of Lingulas and Orbiculas ( Falls of St. 

 "Croix)." 



At page 94, speaking of the strata above low water of the Mississippi 

 near the mouth of Black river, and Mountain island, Dr. Owen says: 

 "Here, in addition to some species of the Genera Lingula and Orbicula 

 " which occur at the Falls of St. Croix, there are some^remarkable forms of 

 "Trilobites, associated with numerous specimens of Obolus." It may be 

 questionable, therefore, whether Mr. Bilungs refers to the Obolus of 

 Mountain island and Black river, or to the Orbicula of St. Croix river, 

 which is not associated with primordial Trilobites. If he refers to the 

 former, then he should have done justice to Dr. Owen by citing his obser- 

 vations. 



Since, however, the author of the Genus Obolella has taken such pains 

 to proclaim to the world that, the little Linyula? jmlita is an Obolella, 

 though mistaking its locality and position, it would be ungenerous to at- 

 tempt to separate it from that, genus. 



