AND INHERITANCE IN PEDIASTRUM. 377 



have been regarded as of subgeneric rank and are the basis of the 

 integrum (Fig. i), Monactinia or simplex (Fig. 2), the Diactinia 

 or two-spined (Fig. 3), the Triactinia or three-spined and the 

 Tetractinia or four-spined series. A tendency to the formation of 

 a four-spined type of cell is shown in the species P. hiradiatiim 

 Meyen, and the peripheral cells of P. tricormitum Borge are de- 

 scribed as having three spines. Braun ('55) recognized the diag- 

 nostic value of cell form in making four sections of the genus, the 

 Monactinia, P. Simplex, Anomopedium, P. integrum (Nag.), the 

 Diactinia, P. Boryanum, etc., and the Tetractinia, P. Ehrenbergii. 



A further class of differences which has been given specific rank 

 is found in the degree of similarity in form between all the cells of 

 the colony. In P. Boryanum (Fig. 4) the interior cells are quite 

 different in form from the peripheral series, while in P. integrum 

 (Fig. i), on the one hand, and P. clathratum (Fig. 3) on the other, 

 the cells are very much alike throughout the colony. 



It seems natural to assume that such forms as P. integrum (Fig. 

 i) with its oval cells, sometimes with two papillae, but with no 

 striking form differentiations, represent the more primitive species, 

 though perhaps P. integrum itself is only an environmental form as 

 I am suggesting in another paper, and that evolution has progressed 

 toward the simplex type with one spine on the one hand, and the 

 two-spined type on the other. The tendency to the development of 

 spines more strongly on the peripheral cells probably came first (P. 

 Boryanum, Fig. 4) and later the development of the strongly four- 

 lobed form in all the cells of the colony (P. clathratum, Fig. 3). 

 Positive evidence that the evolution of the group has followed this 

 course is, however, lacking. The origin of the tendency to pro- 

 duce spines is also not obvious. We shall find a direct relation 

 between the cell form and the intercellular relations in the colony 

 in comparing the colonies of P. Boryanum (Fig. 4) and P. asperum 

 (Fig. 5) and we have evidence for the assumption that the result 

 of a direct environmental influence has been transformed into an 

 inherited cell character in such cases but no support for such an 

 hypothesis is found in the three-spined cells of P. tricornutum 

 Borge or the four-spined cells of P. hiradiatum Meyen. 



As noted, the number of cells in a colony has been shown to 



