OF CREATION. 243 



ceous period. The Terebratulae (fig. 101) were also 

 abundant during the whole period, and continued 

 quite to its close. 



The univalve shells of this period are also interest- 

 ing. Of those nearly allied to existing forms, the 

 annexed figure (102) is a good example. Fig. 102 

 But the most remarkable, beyond all com- 

 parison, are the indications of the ancient 

 Cephalopoda at this the last point of their 

 development. These animals, having been 

 developed throughout the lias and oolitic 

 period in extreme abundance, were conti- 

 nued in the cretaceous rocks, and there TORNATELLA. 

 seem to have expanded into a vast multitude of 

 strange forms before becoming finally extinct. 



The forms thus assumed were curious, and the 

 exact bearing of them upon the habits of the animal 

 it is by no means easy to recognise. The most sim- 

 ple form in which an animal forms a habitation con- 

 sisting of a number of compartments may be under- 

 stood by examining any univalve shell. The greater 

 part of the animal is inclosed in a muscular sac called a 

 mantle, capable of depositing carbonate of lime. As 

 soon as one coat is deposited, which, of course, as- 

 sumes the shape of the muscular mantle, the simple 

 shell is perfected. If, as the animal grows, it is 

 developed in a spiral form, the shell increases at the 

 aperture ; but if the extremity does not adapt itself 

 to the original shell, and remain always of the same 

 size, it must, as it increases in size, withdraw itself 

 from its former compartment, and build a wall of par- 

 tition, and in this way we have the first step towards 

 the formation of the shell of the ammonite or nau- 



M 2 



