424 STUDIES IN EVOLUTION 



the parent cell. Old-age characters are expressed in two 

 ways: First, the cell walls become thickened around the 

 margin of the epitheca without destroying the symmetry of 

 the corallum, as shown in Plate XXX, figure 2; second, by 

 the indefinite and unequal development of the peripheral 

 cells, together with the addition of calices budding from the 

 cells forming the primaty circle. One specimen, appearing 

 at first sight as an example of cell division or fission, is 

 shown in Plate XXXI, figure 1. It may be explained as 

 resulting from the abnormal growth of the second and adja- 

 cent calices, four and eight. This lateral impulse further 

 resulted in sending off the small, peripheral, tertiary coral- 

 lites numbered in the figures 9, 10, 11, 12, and 13. 



It should be understood that this arbitrary expression of 

 normal and abnormal growths applies only to the species 

 Pleurodictyum lenticulare. The same numerical arrangement 

 will not hold good for genera like Favosites, Michelinia, 

 Striatojyora, etc. Otherwise, it is believed, the general laws 

 of growth here brought out will hold good for these and 

 other related genera. 



Some doubt may exist as to the propriety of referring the 

 specimens illustrated on Plate XXVII, figures 9-11, to 

 P lenticulare. Unfortunately, material of this kind is rare 

 and difficult to obtain. With the exception of the position 

 and direction of the first bud (figure 10), all the characters 

 agree, so far as can be observed, with ordinary specimens of 

 P. lenticulare. The second cell of the corallum represented 

 in Plate XXVIII, figure 1, curves rapidly backward, al- 

 though at first the axis has an anterior direction. Taking 

 this view of the specimen (Plate XXVII, figure 11), it is 

 not difficult to see how the succeeding enlargement and 

 curvature of the bud could extend backward, thus properly 

 limiting tbe size of the eighth or last of the primary circlet 

 of calices. 



The method of determining the relative age and succession 

 of the corallites can be seen in Plate XXVIII, figures 1, 2, 

 and Plate XXIX, figure 2. The initial cell occupies the 



