240 MOLLUSCA. 



and, in many cases, intimately united with the substance of 

 the beds ; the shells belonging to the newer members are 

 much less altered in their form and texture, separate more 

 readily from the surrounding rocks, and appear like recent 

 shells somewhat weathered. The shells are found in nearly 

 all the different kinds of rock, but are more numerous in 

 the calcareous strata. In the recent or superficial strata, 

 fossil shells are frequently to be met with. The species 

 which here present themselves bear so close a resemblance 

 to the existing kinds, that conchologists are disposed to con- 

 sider them as the relics of animals which still exist. In 

 many cases, the prototypes may be found on the neighbour- 

 ing shore or lake, but in other instances they must be sought 

 for at a greater distance. These shells are found in beds 

 of gravel and sand, and likewise in great abundance in shell 

 marl. 



It appears, then, that the shells in the older strata differ 

 specifically from those which the newer strata contain ; and 

 that they have belonged to molluscous animals, which no 

 longer exist in a living state on this globe ; that, in the 

 new r er strata, the fossil shells bear a closer resemblance to 

 existing species ; and that in the last formed strata, remains 

 of species actually existing are to be met with. 



In this geological distribution of the remains of testace- 

 ous animals we may likewise perceive that, in the older 

 strata, the inequivalved shells are more numerous than the 

 other kinds; and that the canaliculated univalves are seldom, 

 if ever, to be met with in the transition or older members 

 of the floetz series, but that they become more numerous 

 in the newer members of the floetz rocks, and in the allu- 

 vial strata. Circumstances of this kind have induced geo- 

 logists to conclude that different formations could be dis- 



