THE MODIFICATION OF GENERIC CHARACTERS. 285 



not a specific character, but a matter of variation amon^ indi- 

 viduals of a given species;" and remark further, " This mo- 

 bility in the loop of Zygospira is without parallel among 

 other genera." * 



This case of the Zygospira loop is a striking example of 

 rapid evolution. It has the appearance of being an insignifi- 

 cant feature, only a variation, because of the presence of all 

 the intermediate variations at the initial stage. 



Relation of the Primary Lamellae to the Crurse. — 2. A 

 second example is seen in the modification of the direction of 

 the primary lamellae after they set out from the end of the 



Fig. 77. Fig. 78. 



Fig. 77.— a Spirifer, showing part of the brachial valve, the brachidium with the primary lamellae, 



the jugum, and the spiral coils. 

 Fig. 78.— Cyrtina, the brachial valve removed, showing the brachidium with the spiral coils 



turning upwards into the produced umbonal part of the pedicle valve. 



crurae. There are two ways in which this direction differs: 

 {(I) The lamellae may proceed directly toward the front of the 

 shell away from the crurae, as in the case of Spirifer and 

 Cyrtina (Figs. 'J'J, 78); or they may, immediately after their 

 origin, take a sudden bend upon themselves, making a twist 

 or double bend before proceeding along parallel to the inside 

 surface of the shell, as in Athyris (see Fig. 79); observe also 

 the brachidium in Figs. 64, 65 (p. 279). The latter is re- 

 garded as a characteristic of the family Athyridae, and, 

 although the family as a whole is the more differentiated and 

 later to be dominant, there are several well-marked genera 



* See ], c. p. 156. 



