1919 Smith; on Costaria 309 



The changes in the side ribs are more irregular than those in the mid- 

 rib. A discrimination may be made between the variations in the number 

 of distinct ribs^ and the apparent splitting or separation of one rib into 

 two or more which remain comjDaratively close together, forming what 

 may be called a rib-group. 



The side ribs show tlie same variations as the middle one, and in addi- 

 tion may form rib-groups. The individual ribs in these groups exhibit 

 the same variations as the side ribs. One frond shows a variation from 

 4 to 7 depending upon the region of the blade at which the ribs are counted 

 (Fig. 8). The most complex venation found is in a fragment of a very 

 large plant in which there are from 7 to 11 ribs (Fig. 9). The midrib 

 and one outer side-rib are constant thruout, but the others form groups. 

 Two of these ribs in a group may coalesce and then separate, after which 

 one or the other may disappear completely. In one group is a small, not 

 very clearly defined rib which extends only 6 cm. and does not reappear 

 again or have any connection with any other rib. 



In one frond an injury near a rib resulted in the almost complete 

 disappearance of the injured rib (Fig. 2). Whether some of the varia- 

 tions, either of suppression or proliferation, may be the result of injury is 

 not known. No explanation for the cause of the variations is attempted. 



