THE STRUCTURE OF THE GRAPTOLITES. 



247 



individuals arise, as usual, in threes. Of these only two open, the 

 gonangium and the theca. Now, since the individuals open in fours, 

 one might expect that these four individuals would belong to two 

 consecutive generations. That, however, is not the case, for the four 

 individuals of each separate twig belong to three different generations, 

 as is shown by a comparison between the above table and the 

 following statement of the components of each twig : — 



Thecas h and i Gonangia a and c Left-hand twig. 

 Z>i ,, 2 ,, «i „ Ci Right „ 



,, ^2 ), 3 » «-2 ,, ^2 Left ,, 



On each twig there are thus two thecae and two gonangia. The 

 gonangia are so placed that the elder always opens like a, the younger 



Fig. 19. — Series of Transverse Sections of Ptilograptus suecicus. 

 For explanation of lettering, see text. 



like c, and thus it is that they always come to lie on the back of the 

 twigs. The two gonangia belong to two consecutive generations. 

 The thecae, on the contrary, that open in any one twig are originally 

 separated by one or two generations. The elder theca opens like b, 

 the younger like i. 



The only real difference between this genus and, say, a Dictyomma 

 will thus consist in the individuals here being kept together for some 

 time, so as then to open in alternate groups, which I call twigs. As 



