172 



Contributions to Palceontology. 



little elevated close to the margin. The marginal rim 



is a little wider in front than at the sides. 



The specimens are casts of the interior in friable sand- 

 stone; 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 ex- 

 tends 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 distinctly 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 fea- 

 tures 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 trilobate form is the pygidium. The extension 

 of the middle lobe so near to the extremity, offers an ob- 

 jection to regarding it as the glabella. 



This species occurs in friable sandstone, with Dikelocejphalus 

 osceota, 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 appa- 

 rently 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, unde- 

 terminable fragments of other Trilobites ; and among them are im- 

 pressions of parts of the head, which is strongly pustulose, and 

 portionr of a pygidium of a very different character from any that 

 have hitherto been noticed. Some fragments of the thoracic seg- 

 ments in the same stone are much larger than any corresponding 

 parts of Dikelocephalus minnesotensis which have been seen in the 

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



