FOSSIL INVERTEBRATED ANIMALS. 445 



as well as the form of their aperture, proves evidently that they 

 have been inhabited by paguri — and yet these last are not 

 found. But, perhaps, it may be that, if any are found, their 

 fragility does not permit them to be gathered, for they are never 

 seen in collections. 



It may be asked, if the oolites, which are met with in strata 

 presenting ammonites, were already formed when the shells 

 existed, or if they were formed at the same time in which the 

 petrifaction of the stratum took place. The state in which 

 these shells are found, as well as the belemnites, may assist us 

 in the resolution of this question. 



Ammonites are found, whose partitions, especially the more 

 recent, are filled with oolites ; but whether the testa of these 

 shells were in a perfectly entire state, does not appear to be 

 clearly ascertained. Considering its fragility, we may suspect 

 that this was not the case ; since that of some of them, which 

 is filled, and which is six inches in diameter, is not much 

 thicker than a sheet of paper. But in those species, whose 

 testa is thicker, and which are well preserved, oolites are seen 

 only in the last compartment, which is always open, and the 

 others are filled with crystals. From all that has been observed, 

 we may conclude that the oolites formed a part of the deposi- 

 tion in which they are seen, before the shells were filled ; and, 

 if it can be imagined that they were formed contemporaneously 

 with the petrifaction, we may beheve that the fluids, which 

 deposited the crystals, did not contain the elements of the 

 oolites, or that it lost them by filtration through the testa, or in 

 passing through the siphon. It is the same with the alveolus 

 of the belemnites, which is filled with oolites when the partitions 

 have been destroyed, but in which they are never found when 

 the latter are entire. 



There are oolites which differ very considerably one from 

 another. In some localities, as in the environs of Caen and 

 Bayeaux, they are round or ovoid ; their surface is shining ; 

 their colour ferruginous, and their strata concentric. A little 



