426 STUDIES IN EVOLUTION 



pora and Syringopora. It is without pores on the portion 

 where the corallites and buds are free, but when these are in 

 juxtaposition at their bases mural pores are developed. The 

 upward growth of the initial cell of P. lenticulare proceeds 

 but a short distance before the circlet of peripheral corallites 

 is completed. Thus at this stage there are at least seven 

 mural pores opening into the primary calyx. If this ten- 

 dency to the formation of numerous buds persists throughout 

 the upward growth of the corallites, the non-development 

 of the buds consequent upon the adjacent living corallites 

 would naturally result in the production of mural pores. 

 The basal epitheca limits the fleshy portion of the organisms, 

 and represents an area unfavorable to the acquisition of 

 food or for the natural development of calices. Therefore it 

 would prevent both the maintenance of mural pores and the 

 growth of basal buds.* 



A Favosites in which one or more cells became inactive or 

 dead shows in its subsequent growth the closing over of this 

 area by the budding of the surrounding cells. Each cell is 

 connected with the parent by an apical pore (Plate XXXI, 

 figures 3, 4). Without this opportunity to bud afforded by 

 the death of one or more corallites, or by their divergence, 

 the adjacent cells would have developed only mural pores. 

 In the figure of Pleurodictyum problematicum given on Plate 

 XXXI, figure 2, three of the initial pores are indicated by 

 dotted lines from p. No distinction can be made between 

 these and the ordinary pores, except that the latter are 

 usually not as large. This difference in size would be ex- 

 pected, as the primary pore represents the bud which suc- 



* The presence of basal mural pores or openings through the epitheca has 

 been asserted by Meek and Worthen {Pal. Illinois, III. 409, 1868). The spe- 

 cimens from which this observation was made, are from a friable sandstone, 

 which does not usually preserve minute details with much distinctness. The 

 depressions between the spinules on the septal lines could easily be mistaken in 

 a cast for the fillings of mural pores, and it is believed by the writer that this 

 interpretation should be given. P. lenticulare occurs as calcareous or silicified, 

 and in the condition of casts. No basal mural pores are present. Also, none 

 can be observed in the casts of P. problematicum, from Pelm, Germany. 



