THE MODIFICATION OF GENERIC CHARACTERS. 283 



Brachiopods (see Figs. 66-72) consists in the fact that the 

 loop or jugum connecting the primary lamella; in the Heli- 

 copegmata sets off from the sides of the lamellae before they 

 have begun to reverse their direction in forming the volution, 

 and the continuation of these lamella; is supplied with a cal- 

 cified spiral support ; while in the Ancylobrachia the connec- 

 tion docs not take place till after the primary lamellae have 

 reversed their direction and are proceeding backward toward 

 the crura. For them there is no calcified continuation of 

 the lamellae, but the brachial arms, although still preserving 



Fio. it. 



Fig. 67. 



Fig. 68. 



Fig. 69. 



Fig. 70. Fig. 71. Fig. 72. 



Figs. 66-72. — Diagrams expressing the relationship between the brachidial apparatus of Ancylo- 

 brachia and Helicopegmata. 66-69. The loop of the Ancylobrachia. D = brachial valve ; 

 V z= pedicle valve ; c = crura ; / = primary lamella of the brachidium ; / = the connecting 

 bar of the loop corresponding to the jugum of the Helicopegmata {b in the lower diagrams) ; 

 a = the fleshy spiral arms, not supported by calcified lamellae. 70 = Brachidiura of Zygo- 

 spira, 71 of Anazyga, 72 of Dayia, seen from the side ; the lettering the same as above, excep*' 

 b = jugura and s = spiral coils of the brachidium. 



the spiral form, from the angle of the loop are entirely fleshy, 

 and therefore not preserved in the fossil state. 



If we examine in detail the kind and extent of modifica- 

 tion exhibited in the various genera, in their relations of the 

 time and order of appearance in the geological faunas, we gain 

 a close view of the actual fact of evolution of new characters, 

 as seen in the following particulars : 



Relation of Jugum to the Primary Lamellae. — i. The posi- 

 tion of the jugum in relation to the point of outset of the 



