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 mineraiogists 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 greywacke 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- 
longedtoone epoch. They are dissimilar to the recent species, 
and no longer exist in a living state. In the newer members 
of this class, such as the gypsum and chalk rocks, the species, 
insome examples, bearamuch closer resemblance to the exist. 
ing races, and several species cannot be distinguished, itis 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, 
