GEOLOGICAL BIOLOGY. 



Geological Range of the Families. The following table of 

 the geological range of the families, subfamilies, and genera 

 will help to give a notion of the time-relations of the forms 

 under discussion. 



TABLE REPRESENTING THE GEOLOGICAL RANGE OF THE FAMILIES AND 

 SUBFAMILIES OF THE HELICOPEGMATA, WITH THE NUMBER OF GEN- 

 ERA AT PRESENT RECORDED FOR EACH ERA. 



Description of the Structure of the Brachidium. The ele- 

 ments of the brachidium in the Helicopegmata are seen with 

 considerable elaboration in the genus Athyris. 



In the interior of the brachial valve are seen in the apical 

 region, proceeding forward from the hinge-plate, two stiff pro- 

 cesses called the crura (Fig. 79) ; attached to the crura, and 

 in Athyris making a short twist toward the base of the crura, 

 proceed two ribbon-like bands toward the wall of the shell, 

 and thence along parallel to its inner surface toward the 

 front : these are thus far called the primary lamellce. At the 

 front, and continuously with these lamellae, the spiral coil 

 begins by curving toward the opposite valve, thence upward 

 parallel with its inner surface to near the crura, thence turn- 

 ing again toward the wall of the brachial valve, and in the 



