134 THE MICROSCOPE. 



quently occur in such great numbers, that the fine calcareous sand 

 which constitutes the sea-shore in many places consists almost entirely 

 of their microscopic coats. At former periods of the earth's history, 

 they existed in even greater profusion than at present ; and their fragile 

 shells form the principal constituents of several very important geologi- 

 cal formations. Thus the chalk appears to consist almost entirely of 

 the shells of these animals, either in a perfect state, or worn and broken 

 by the action of the waves; and they occur in great quantities in the 

 marly and sandy strata of the tertiary epoch. The stone which is uni- 

 versally employed in Paris as a building stone is almost entirely 

 composed of the fossil shells of an animal belonging to the order 

 MUiola. 



In the Stichostegidae the chambers are placed end to end in a row, 

 so as to form a stsaight or but slightly curved shell. In the second 

 family, the Enallostegidce, the chambers are arranged alternately in two 

 or three parallel lines ; and as the construction of the shell is always 

 commenced with a single small chamber, the whole necessarily acquires 

 a more or less pyramidal form. The third family, the Helicostegidce, 

 presents us with some of the most beautiful forms that we meet with 

 in these shells. They commence by a small central chamber ; and each 

 of the subsequent chambers, which are arranged in a spiral form so as 

 to give the entire shell much the aspect of a minute flattened snaU, is 

 larger than the one preceding it. It is in this family that we find the 

 nearest approach, in external form, to the large chambered shells of the 

 cephalopodous moUusca, of which the nautilus and the argonaut are 

 examples. The fourth family, the Untomostegidce, stand in the same 

 relation to the preceding as the EnallastegidcB to the Stichostegidm ; 

 that is to say, the chambers are also arranged in a spiral form, but in a 

 double series. A fifth family includes those shells in which the cham- 

 bers are arranged round a common perpendicular axis in such a manner 

 that each chamber occupies the entire length of the shell. The orifices 

 of the chambers are placed alternately at each end of the shell, and fur- 

 nished with a curious tooth or process. The MUiola will serve as an 

 example of this family. 



It is probable, although by no means certain, that the animals whose 

 fossil shells, termed WnmmvMtes, are found in great quantities in the 

 chalk and lower tertiary strata, are also to be regarded as members of 

 this class. No living example of this form of animal has yet been met 

 with ; but in a fossil state whole mountains consist almost entirely of 

 their shells. 



The great Pyramid of Egypt, covering eleven acres of ground, is 



