132 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



Mr. Billings has given, of one of tlie species of tlie genus, the accom- 

 panying illustration of " the inteHor of one of the valves, supposed to he the 

 ventral, showing the muscular impressions.'''' This will afford the means of 

 comparison with O.f polita, figures 20 & 21 on Plate 6. 



ObolelJjA chrowatica ( BilliBg&) . 



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

 river in the Western States", and which be has identified with Lingular 

 polita, has, according to that author, "^the central scars" "close together, 

 " one on each side of the median line and parallel ". 



It becomes necessary, in this connexion, to notice the Genus Spondy- 

 LOBOLUS of M'CoT, published in the Annals Nat. History, 2d series. 

 Vol. viii, p. 407 ; and in British Pala&ozoic Fossils, p. 255, 1855, 



The following is the description of that genus : 



sufficient to suggest some discussions on its relations to that genus, witliont a hint 

 from any one. 



Nevertheless, the species cited by Mr. Billings, if the locality be correct^ is. clearly 

 not the one described by me j for it does not occur at the Falls of St.Croix, nor on the 

 St.Croix river " in theTVestern States,-^' so far as I know ; nor am I able to find, in Dr. 

 Owen's Report, anyreference of such a form to the Falls of St.Croix : And furthermore 

 I cannot find itanywhere stated by Dr. Owen that the Lingula and Orbicdla (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 locality. Dr. Owen distinctly describes the 

 " First Trilobite bed" as composed of "Ferruginous Trilobite grits, Schistose sand- 

 •' stone containing forktailed Trilobite beds and Obolus layers, 1-8 feet; Magnesio- 

 *^ calcareous rock, with Obolus and forktailed Trilobite, E feet." The typical locality 

 of these beds is at Mountain island, and they are placed by Dr. Owen above the " high- 

 " ly fossiliferous, schistose, silico-calcareous layers, interlaminated with argillaceous 

 " marly beds charged with sulphate [sulphuret] 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 maybe questionable, therefore, whether Mr. Billings 

 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 observations. 



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

 to the world that the little Lingula? polita is an Obolella, though mistaking its 

 locality and position, it would be ungenerous to attempt to separate it from that 

 genus,; 



