284 



GEOLOGICAL BIOLOGY. 



primary lamellae from the crurae and the point of their turn- 

 ing back to form the first volution of the spiral, is perhaps 

 one of the most fundamental differences, as it affects the 

 whole mode of elaboration and position of parts of the brachid- 

 ium. The extremes possible are for the jugum (i) to join 

 the lamellae immediately at their origin from the end of the 

 crurae, and (2) to be situated at the extreme front of the shell 



Fig. 73. 



Fig. 74. 



Fig. 75. Fig. 76. 



Figs, 73-76. — Zygospira ntodesta. (After Clarke.) Showing the variation in the position of the 



jugum. 



joining the lamellae where they begin to turn back to make 

 the first volution of the shell (compare Figs. 75 and 74). 



The rapidity with which the differentiation of structure in 

 this particular took place is seen in a remarkable way by the 

 examination of the earliest representatives of the Helico- 

 pegmata, as illustrated by the diagrams of the form of the 

 brachidium of Zygospira niodcsta and of the closely allied form 

 Z. putilla prepared by Mr. Clarke.* 



The position of this jugum (or loop) is regarded by Hall 

 and Clarke as of less than specific value. They say, " This is 



* Pal. N. Y., vol. vni. pt. 2, fasc. i. pp. 155 and 157. 



