. V. ARRANGEMENT. 193 



Univalves (spiral or invo- 

 luted) the body whirl united to one or 

 more of the succeedin?; whirls In sim- 

 ple univalves, the whole of the shell. 



ERISMATOLITHFS various, e. g. 



In reliquia of cellular fulciments, the 

 stirp exhibiting the internal form or 

 structure of the cells. 



solid Fulciments,, the stirp 



exhibiting its external form. 



PHYTOLITHUS the leaf (includ- 



it will scarcely be disputed, that the parts selected are those most 

 eligible for our present purpose. With respect to Phytolithus, a 

 reason must be assigned, why the parts of fructification, (by 

 which doubtless the natural tribes or orders of the originals might 

 best be determined) are rejected, and the leaf and stem substituted 

 in their place To those acquainted with the subject of extraneous 

 fossils, it is well known \\rnt flowers and fruits are so rare among 

 the Phytolithi, that to have made them the foundation of perma- 

 nent species, would have been nearly the same, as excluding per- 

 manent species from the genus. Indeed, the fructification is so 

 minute in one tribe of plants, namely the Filices, from which the 

 most numerous, beautiful, and perfect specimens of vegetal reli- 

 quia originate, that it is seldom if ever visible in the fossil species, 

 and of course is useless in their discrimination. The leaf, there- 

 fore, is the part to be preferred, as that from which by far the 

 greatest number of permanent species may be formod It is the 

 part, also, from which the most certain specific differences may be 

 drawn; and will be found, in conjunction with the stem, suffi- 

 ciently indicative of the natural tribes or orders of plants at 

 least, as far as these distinctions are applicable to the fossil sub- 

 jects. 



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