116 C.J. A. Meyer — 3Iicras(ers in the English Chalk. 



Arrange them, as Nature has done, strati graphically, and one cannot 

 but allow of certain distinctions not otherwise so readily apparent. 

 In the one case, however, one will have, unintentionally, grouped 

 together specimens in which the position of the mouth, or peristome, 

 will be as different as appears in the outline figures 1 and 2. 

 Fig. 1. Fig. 2. 



In the other, or natural, arrangement, the position of the mouth 

 will be found to be almost constantly the same in specimens from 

 the same geological horizon, and widely different only between 

 those from different horizons. And it will be observed that a cor- 

 responding variation takes place in respect to the position of the 

 apical disk. In the natural arrangement of the specimens one 

 obtains, in fact, an almost regular gradation in the relative positions 

 of the mouth and apical disk (as shown in Fig. 3), and a gradation 

 also, more or less regular, in other minor points of difference. 



b a 



Fig. 3. 



Fig. 3. — In which the changing relative positions of the mouth and apical disk 

 are indicated by the lines a-a, b-b, etc. 



i 



