974 



PRINCIPLES OF STRATIGRAPHY 



contain adult leaves and sometimes entire branches in which the 

 leaves never pass beyond the four-lobed or even two-lobed type. 

 By retardation, the ancestral characters are retained side by side 

 with the normal characters of the modern species. Eight-, lo-. 12-, 

 and sometimes 14-lobed leaves also occur, evidencing local intra- 

 colonial acceleration. 



An example among colonial animals w'ill further make this 

 clear. Favosites canadensis of the Onondaga limestone is charac- 

 terized by small corallites with polygonal openings, while among 

 these are scattered at regular intervals larger ones with nearly cir- 

 cular openings. Favosites placenta of the middle Hamilton of 



Fig. 258. Portions of the surfaces of two colonies of Favosites. somewhat 

 enlarged, to show intracolonial acceleration, a. F. canadensis, 

 showing the regular distribution of the large cylindrical corallites 

 among the smaller prismatic ones. b. F. placenta, showing the 

 F. canadensis condition on the left, and the large prismatic 

 corallites on the right. 



Canada when young (nepiastic) has the characteristics of adult 

 F. canadensis, but when colonially adult it has retained the F. cana- 

 densis type in certain portions of the colony only, other portions 

 having developed a uniform series of large corallites with polygonal 

 apertures. Here, then, certain portions of the colony have been 

 retarded in their development, retaining ancestral characters, while 

 others have continued to develop. In the upper Hamilton a species 

 of the F. placenta type (F. placentoides) occurs with all the coral- 

 lites uniform, large and angular in aperture (Fig. 258). 



Acceleration in certain portions and retardation in others when 

 occurring in more or less regular manner result in the formation of 

 colonial ornamentation, such as is so characteristic of Bryozoa. 



Atavism or Reversion. Not infrequently among a large number 

 of individuals of a species characterizing a certain horizon a few 

 will be found in which ancestral characters occur in the adult, thus 

 recalling species of a lower geologic horizon. This atavism or 



