62 



Leptocyathus elegans (Haeckel) may be mentioned as a representative 



of the group C. 



Of the groups Cj, which of course give only a series of new forms 



for even values of n, no 

 instances among plants 

 and animals have been 

 found up till now. 



As an illustration of 

 some polyhedra of this 

 kind, in fig, 72 and 73 

 the crystal-forms are 



<"". . 





Fig. 71- 

 Fruit of Badamia Commersoni. 



reproduced of scheelite : 

 CaWOt, (C H 4 ), and of 

 apatite: Ca 5 Cl (P0 4 ) 3 , 

 (C^f ) ; these figures 



show the respective symmetries rather clearly. Of course the hete- 



ropolar character of the principal axis has here 



disappeared; and from the figures reproduced 



it may be obvious that the polyhedra under 



consideration possess really a symmetry-centre. 

 The symmetry of the group C*j is very often 



met with in the case of crystalline substances: 



all so-called monoclinic substances, the number 



of which is extremely great, belong to this 



group, as far as they are holohedral. 



Commonly the horizontal plane of symmetry 



is placed vertically in figures of this kind, so 



that the binary axis now will have a horizontal 



direction. This 



Fig. 72. 

 Scheelite. 



Fig. 73- 

 Apatite. 



Let us start with those 



custom is followed 

 also in the accompanying drawing 

 (fig, 74) which represents a crystal 

 of the mineral amphibole'. 

 p Ca(Mg,Fe)(SiO s ) z + q MgAl 2 Si0 6 

 in various proportions p and q. 



8. The remaining groups of the 

 second order yet to be dealt with, 

 are related to the dihedron-groups D n , 

 or to the endospherical groups T, 

 K, and P respectively, 

 which are related to D n> and whk 



