180 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



The specimens are casts of the interior in friable sandstone ; 

 and the finer markings, and even any marks of furrows, unless 

 well defined, would not be preserved. 



A pygidium? in the same specimen of sandstone is trilobate, 

 a little wider than long. The trilobation extends nearly to the 

 posterior extremity, and is separated from it only by a narrow 

 border. The middle lobe is fully once and a half as wide as the 

 lateral lobe, somewhat flattened on the summit, and very distinct- 

 ly limited by the dorsal furrows. 



In one specimen, the axis appears to be annulated ; but in 

 specimens so minute, when the accession or removal of a grain of 

 sand may alter the form and characters of a fossil, it is not easy 

 to decide in regard to the minor features of a species. 



It is with some hesitation that I refer the separated parts to 

 the same species ; nor can it be decided positively that the tri- 

 lobate form is the pygidium. The extension of the middle lobe so 

 near to the extremity, offers an objection to regarding it as the 

 glabella. 



This species occurs in friable sandstone, with Dikelocephalus osceola, at 

 Osceola mills on the St. Croix river. 



Fig. 25. The head?, four times enlarged. 



Fig. 26. The pygidium? enlarged in the same proportion. 



Fig. 27. A specimen enlarged to the same degree; the middle lobe apparently marked 

 by transverse furrows. 



The few specimens of a dark rusty-colored sandstone from Osceola mills 

 have proved very prolific of species. Besides those already enumerated, there 

 are, in the same sandstone specimens, undeterminable fragments of other 

 Trilobites ; and among them are impressions of parts of the head, which 

 is strongly pustulose, and portions of a pygidium of a very different cha- 

 racter from any that have hitherto been noticed. Some fragments of the 

 thoracic segments in the same stone are much larger than any correspond- 

 ing parts of Dikelocephalus minnesotensis which have been seen in the 

 collections, and perhaps belong to a species of that genus. 



Whenever this locality, and the region about it, shall he more fully in- 

 vestigated, we may confidently predict that additions of much value and 

 interest will be made to the primordial fauna of the Upper Mississippi 

 valley. 



