GEOLOGY. 313 



this lingula grit a species of obolus, very much like those obtained 

 from the lowest sandstones of Russia ; and in similar beds, ten miles 

 below Mountain Island, I discovered a remarkable trilobite, provided 

 with spines, Avhich project backwards from the pygidium. The com- 

 bined labors of the succeeding year (where minute stratigraphical and 

 paleontojbgical sections were undertaken) developed beneath the Lower 

 Magnesian limestone at least six different trilobite beds, separated by 

 iivv.i 10 to 150 feet of intervening strata. 



Tho Menominie trilobite grit, situated about forty or fifty feet above 

 the lowest trilobite bed, is characterized by a minute species of trilobite, 

 chiefly remarkable on account of a spinous appendage, which has its 

 origin at an angle in the middle of the posterior part of the glabella, 

 and projects upward and backward in the median line of the body. 

 The glabella in this species has only one distinct transverse furrow, 

 immediately in rear of the origin of this spinous process. To convey 

 an idea of the abundance of this species in the grits of the Menominie 

 river, it may be stated that, on a specimen measuring three inches 

 square, more than a hundred individuals can be counted. 



In 1849, Dr. Shumard found in green sandstones above the level of 

 the Mississippi, near the head of Lake Pepin, and also on the Wis- 

 consin river, the remains of Crinoidea. This encrinital bed lies some 

 forty feet below the St. Croix trilobite bed. Specimens of both Orthis 

 and Spirifer occur at a lower level, between these crinoidal beds and 

 the Mineska trilobite grits. A fuller description of these various spe- 

 cies of trilobites, which belong, probably, to more than one genus, is in 

 course of preparation for the final report on the geology of the North- 

 west. The object of this notice is merely to show, in connection with 

 the specimens exhibited, that Crustacea do actually occur in Wisconsin, 

 Iowa, and Minnesota, even as low down in the unaltered sedimentary 

 strata as any organic relics have yet been traced ; and that the oldest 

 sandstones of that country contain also Spirifer, Orthis, Obolus, and 

 remains of Crinoidea, besides Lingulas and Orbiculas. 



FOSSILS OF THE SILURIAN EPOCH. 



AT a meeting of the Boston Natural History Society, January, 1852, 

 Professor Rogers stated that Mr. Salter had recently demonstrated in 

 London the fallacy of Hugh Miller's argument for the deterioration of 

 species, based on the supposed existence of fishes, in some of the older 

 rocks, of a higher organization than those of a more recent period. One 

 of the specimens from which the argument was drawn, was shown to 

 be a piece of a trilobite, and another a zoophyte. Prof. Rogers remarked 

 that the only evidence remaining of vertebrata in the Silurian system, 

 was a single occurrence of foot-prints in the sandstone of the St. Law- 

 rence Valley. With regard to the nodules, like coprolites, occurring in 

 the Barlow limestone, Professor Rogers suggested that the same source 

 which could supply phosphate of lime to coprolites, might supply it in 

 the form of concretions to the geological formation itself; so that the 

 occurrence of such nodules merely was not positive proof of the exist- 

 ence of fishes at that time. 



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