MOLLUSCA. 239 



In those ancient strata upon which all the others are in- 

 cumbent, and which are called primitive, no remains of 

 shells, or other relics of organized bodies, have hitherto been 

 detected. These rocks are therefore supposed to have re- 

 ceived their arrangement previous to the creation of animals 

 and vegetables, or to have been so much altered as to have 

 all traces of organisms obliterated if such existed. In that 

 group of rocks which rests upon the primitive strata, and to 

 which mineralogists give the name of transition, fossil shells, 

 as well as the remains of vegetables, have been observed. 

 The shells exhibit such striking peculiarities of form, and 

 bear so remote a resemblance to the recent kinds, that they 

 are considered as the remains of species which do not now 

 exist in a living state on the globe. They are much chang- 

 ed in their texture, and in general intimately united with 

 the contents of the stratum. They are chiefly found in the 

 beds of limestone, sometimes also in the grey wacke and clay 

 slate. In the numerous and ill-characterised series of strata 

 which are incumbent on the transition class, and to which 

 some mineralogists attach the term floetz, the remains of 

 shells are much more numerous. In the older members of 

 this class, such as the red sandstone and independent coal 

 formations, the shells, though in a few instances different in 

 form from those of the preceding class, appear to have be- 

 longed to one epoch. They are dissimilar to the recent specie?, 

 and no longer exist in a living state. In the newer members 

 of this class, such as the gypsum and chalk rocks, the specie?, 

 in some examples, bear a much closer resemblance to the exist- 

 ing races, and several species cannot be distinguished, it is al- 

 leged, from them, by any satisfactory characters furnished by 

 the shell. The fossil species found in the rocks of the older 

 members of the class are greatly altered in their texture, 



