576 ON THE FORMATION OF ROCKS. (June^ 



Neptunian of the 1st class. They contain no remams of 

 organic matter, nor many particles of rocks rounded by 

 attrition; nor do they resemble any of the precipitations 

 or depositions contained in the first order of the Neptu- 

 nian rocks. They are allied to the Neptunian division, 

 by their stratification and relative position; ^touching and 

 perhaps alternating, with some rocks of the 2d order of 

 the Neptunian division; in structure and external appear- 

 ance, they resemble many of the members of the volca- 

 nic family, and approach in many of their properties to 

 that species or formation of Rocks, called by Werner, 

 the Trap or Basalt formation, (newest flcetz trap forma- 

 tion). The few remarks we have to make on this order, 

 may therefore come most properly after we have examin-. 

 ed all the rocks united either to the Neptunian or volca- 

 nic origin, by positive or rational analogy. This will 

 clear the field; and bring those of doubtful and conjectu- 

 ral origin, into a smaller compaSs, where their resem- 

 blance or difference can be better examined; and the re- 

 suit of the comparison may throw light on the 3d class 

 of Rocks, where positive or rational analogy is deficient, 



(To be continued.) 



Account of two new genera^ and several nexv species, of 

 freshwater and land shells. By Thomas Say, Read 

 May2S, 1818. 



Genus ^POLYGYRA. 



Shell discoidal, more or less carinated on the upper 

 ^dge of the whorls, umbilicated; aperture longer than 

 broad; lips thickened, toothed, or folded and continued, 



