34 LOWER PENINSULA. 



J 



Emmonsii. Pores large and moderately numerous. The ter- 

 minal parts of the stems are always formed of comparatively thin- 

 walled, regularly formed tube orifices. On the lateral faces of the 

 stems the orifices are often considerably narrowed and disfigured 

 by incrassation of the tube walls, while the lateral pore channels 

 retain their usual diameter, and become transformed into long 

 vermicular ducts of nearly equal size with the principal tube chan- 

 nels. Such specimens are very unlike, in external appearance, 

 those with normally formed tube orifices. 



I have united within this species two forms, which differ from 

 each other in the size of the tubes and of the stems, but otherwise 

 correspond in structure. The form with larger stems and coarser 

 tubes is found in the upper Helderberg limestones of Michigan, 

 but particularly well preserved in the limestones of the Falls of the 

 Ohio, and at Charleston Landing, in Indiana. The smaller form 

 of more delicate structure is peculiar to the Hamilton group, and is 

 found at Thunder Bav, often also in the drift associated with other 

 Hamilton fossils. 



Plate XH. — Fig. i represents a silicified specimen from the Falls 

 of the Ohio, with unusually well-preserved surface characters ; the 

 central figure in the second row of the plate is a similar specimen of 

 smaller size, and the two stems to the left of it are silicified speci- 

 mens of the smaller variety, found in the Hamilton strata of 

 Alpena. To the right of the central figure the upper fragment 

 represents a horizontal section through a specimen formed of tube 

 casts, narrowed by incrassation of the walls, and connected by radiaj 

 branches, which are the casts of the lateral pore channels. The 

 lower figure is a vertical section through a similar specimen, in the 

 peripheral parts likewise presenting the tube casts in a lateral 

 view ; the centre of the specimen presents not casts, but the actual 

 tube channels, intersected by transverse diaphragms, and in unal- 

 tered, not incrassatc condition. The outer lower stem is a speci- 

 men with abnormally incrassatc tube walls, to show the contrast in 

 external appearance of various modifications of the species. 



FAVOSITES LIMITARIS, N. Spec. 



Ramified and reticulated stems, from five to fifteen millimeters 

 in thickness, forming horizontally explanatc expansions or erect 



