376 THE MICROSCOPE. 



Englypha it is of an oval or flask-like form, with the 

 opening at the smaller end, and the shell appears asr 

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

 In Difflugia^ Fig. 205, A B, the shell is often globular. 

 Rhizopods which never develop more than one chamber 

 or loculus are classed as Monothalamia. 1 



The Polythalamia, or Multilocular Rhizopods, in their 

 earliest state, are unilocular ; but, as the animal increases,, 

 successive chambers are added in a definite pattern 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 constituent 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 

 rnarly and sandy strata of the Tertiary epoch. 



In the Stichostegidce the chambers are placed end to- 

 end in a row, so as form a straight 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 neces- 

 sarily 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, 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 snail, 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 cepha- 

 lopodous mollusca, of which the Nautilus pompilius is an 

 example. The fourth family, the Entomostegidce, stand in the 

 same relation to the preceding as the Enallostegidoe to the 



(1) Diffiugia and Ancella form a connecting link between the naked forms, 

 Amoeba, Actinophrys, &c. and the shell-bearing Rhizopods, Lagena striata, &c. 



