9 2 



THE DARWINIAN THEORY 

 Fig. 14. 



Shells »/■ Peneroplis and of Orbitolites, members of the group of Porcella- 



nous Foraminifera, illustrating the mode of transition from 



the spiral to the discoidal shell. 



A. — An adult Peneroplis shell. The shell is spiral and chambered, the 

 later-formed chambers being very wide, and having a tendency to overlap 

 the preceding ones. x 20 



B. — A young Orbitolites shell, in the Peneroplis stage of development. 

 The shell is spiral and chambered, the last-formed chambers having a 

 more marked tendency to overlap the preceding ones than in Peneroplis. 

 The last or marginal chamber in the specimen figured extends almost the 

 whole way round. x 30. 



C. — An adult Orbitolites shell, seen edgeways, so as to show the thickness 

 of the disc, and the marginal pores through which the pseudopodia are 

 protruded during life. x 7 



D.— An adult Orbitolites shell, seen full face. In the centre is the spiral 

 nucleus, which is the oldest part of the shell, and was originally the only 

 part present ; then comes a part in which the successive chambers become 

 wider and wider, and overlap the older part of the shell more and more 

 completely, and finally the marginal and latest formed part, in which each 

 chamber is circular and completely surrounds its predecessors. 



The fainter radial lines indicate the secondary partitions by which the 

 chambers are subdivided. x 7 



