442 FOSSIL INVERTEBRATED ANIMALS. 



hold together by their parts dove-tailed, seem to be articulated^ 

 They are never invested with crystals, like the ammonites, of 

 strata more ancient than the chalk. In the paste, or substance 

 of the stratum which fills the last of their partitions, there are 

 found a prodigious quantity of little shells, or debris of poly- 

 paries and other marine bodies. The same is the case with 

 the other partitions when the mould is not perfect; which would 

 lead us to believe that, in this instance, the testa has been 

 destroyed on one of its sides. But, with respect to the moulds 

 which are perfect in their circumference, and which may be 

 supposed to have been formed in complete shells, that of each 

 partition is composed of very fine paste, without any mixture 

 of organic remains — the marginal siphon having been too nar- 

 row to admit them to pass. 



These remarks are equally applicable to the ammonites, 

 which are often found with their testa, though still more fre- 

 quently without it. In the first instance, it often happens that 

 the last compartment is filled with the paste formed by the 

 stratum in which they have been deposited ; and that the other 

 compartments are filled with a fine paste, or merely invested 

 with crystals. In this case, we see that the fluid in which this 

 stratum has been found contained two distinct substances — 

 namely, the opaque matter of the stratum, and that which, 

 being filtrated through the testa, or through the siphon, has 

 formed the crystals, and furnished the crystallization which has 

 hardened the stratum. It may be supposed that the animals 

 which inhabited these shells were able to live in the waters 

 which held in solution the substance of the crystals ; for, when 

 they were abandoned, this did not fall, or remain at the bottom 

 of the sea, but after having been filled with the water which 

 surrounded them ; and it is difficult to believe that this water 

 was expelled by any other which could have deposited the 

 crystals in question. 



Certain ammonites having been filled by quartzose sand, their 

 internal mould is formed of sandstone, and what remained of the 



