THE MODIFICATION OF GENERIC CHARACTERS. 28/ 



primitive form of spiral was with few volutions; but if this be 

 the fact, the rapidity of their increase to the extreme, found 

 in Atrypa and in some of the Spirifers (Fig. 77), was early 

 reached in the basal fauna of the Upper Silurian, and it is 

 observed that the embryonic forms have fewer coils to the 

 spiral than the adults (Beecher). (Compare Protozyga (Hall) 

 and Cyclospira with Atrypa reticularis or Spirifer.) 



FIG. 80. Diagram representing the various positions of the spiral coils in the brachiaium of the 

 Helicopegmata. The diagrams are drawn as transections viewed from the beak of the shell, 

 the brachial valve being the upper and the pedicle the lower lines of each figure. A , the posi- 

 tion with apices of the cones directed outward, as in Spirifer ; B, apices directed toward the 

 pedicle valve ; C, apices directed toward the centre of the pedicle valve ; Z>, apices nearly 

 meeting on the median plane ; , apices directed obliquely inward toward centre of brachial 

 valves ; /% apices directed toward the pedicle valve with subparallel axes. 



Direction of the Axes of the Spiral Cones. 4. Among the 

 earlier representatives we have also every position of the 

 spirals, so that the direction of the pointing of the axes and 

 the apices of the cones reaches its full elaboration very early. 

 In Zygospira the apices are directed obliquely toward the 

 centre of the brachial valve (Fig. 80, ) in Atrypa and 

 Atrypina toward the deepest part of the brachial valve (F), 

 while in Spirifer and several other genera they are directed 

 toward the outer margin of the two valves (A); in Coelospira 



