PROTOZOA. 269 



opening at the smaller end, and the shell appears as though 

 formed of a sort of mosaic of small horny pieces. In 

 Difflugiathe shell is often globular. Fig. 162, A B, Rhizo- 

 pods, which never develope more than one chamber or 

 loculus, are called Monothalamia. 



The Polythalamia, or Multilocular Rhizopods, in their 

 earliest state, are unilocular; but as the animal in- 

 creases, successive chambers are added in a definite pat- 

 tern for each family of the order. They all inhabit the sea, 

 and frequently 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 geological 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 ; they occur again in great quantities in the 

 marly and sandy strata of the Tertiary epoch. The stone, 

 which is universally employed in Paris as a building stone, 

 is almost entirely composed of the fossil shells of those 

 belonging to the order Miliola. 



In the Stichostegidce the chambers are placed end to end 

 in a row, so as to form a straight or but slightly curved 

 shell. In the second family, the Enallostegidce, the cham- 

 bers 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 it is possible to meet with in shells. They com- 

 mence by a small central chamber ; and each of the sub- 

 sequent chambers are arranged in a spiral form so as to 

 give the entire shell much the aspect of a minute flattened 

 snail, is larger than the one preceding it. It is in this 

 family that we find the nearest approach, in external form, 

 to the krge chambered shells of the cephalopodous mol- 

 lusca, of which the nautilus and the argonaut are examples. 

 The fourth family, the JEntomostegidce, stand in the same 

 relation to the preceding as the Enallostegidce to the 



