﻿46 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



The increasing space that is divided among the zooecia as periph- 

 eral growth proceeds is variously employed in the complete devel- 

 opment of ramose zoaria. In certain types, particularly simple 

 Integrata, the normal width of the zocecial cavity is maintained 

 solely by deposits of laminated calcareous tissue on the inner sides 

 of the walls. In other types the excess space is in a greater or less 

 degree accounted for by triangular or quadrate open spaces left 

 between adjoining zooecia, and in these the deposit of tissue on the 

 inner side of the walls is correspondingly thin. In other types, again, 

 a part of the space is taken up by a thickening of the original walls 

 in the process of their amalgamation. Finally, in the more complex 

 Amalgamata, the normal width is maintained partly by the thickening 

 incident to amalgamation, partly by deposit on the inner side, and 

 partly by the development of acanthopores and mesopores. 



In Rhombotrypa quadrata the excess space is taken up by deposit 

 on the inner side of the walls, in R. subquadrata and R. crassimu- 

 ralis, mostly in the same way but also by interzocecial spaces, while 

 in the undescribed Richmond and Niagaran species it is chiefly by 

 such spaces. 



The relative amount of excess space varies with three factors : 

 (1) The diameter of the zoarium ; (2) the width of the peripheral 

 zone; and (3) the angle at which the tubes proceed to the surface 

 after bending outward from the axial region. These in turn deter- 

 mine the thickness of the zocecial walls at the periphery and the 

 number of the mesopores. In a narrow branch the increase is pro- 

 portionately more rapid than in a larger stem, in a wide peripheral 

 zone the increase continues for a longer time, and when the zocecial 

 tubes turn abruptly into the peripheral zone and proceed in a direct 

 line toward the surface the increase obviously is more rapid than 

 when the angle is less abrupt. It has probably impressed others, as 

 it has us, as a singular fact that ramose zoaria do not exceed a cer- 

 tain maximum diameter for each species. Primarily it is because no 

 new zocecia are developed after they have fully entered the peripheral 

 zone. Next it depends upon a definite proportion between the respec- 

 tive sizes of the axial and peripheral zones. This proportion varies 

 with the species, those with thin-walled zocecia having a propor- 

 tionally narrow outer zone or slowly curving tubes, and those with 

 thick walls having a correspondingly wider peripheral zone. Finally, 

 it depends upon the capacity of the zocecia of a particular species to 

 take care of the increase in space allotted to it as growth proceeds. 

 When a certain limit, which varies with the species, is reached, 

 growth must cease, and that part of the zoarium dies. But when 



