176 



ACTINOZOA. 



13th, I4tli, 15th, 1 6th, and 17th orders. In but 

 few corallites does a seventh cycle occur. 



The lines in the subjoined diagram are supposed 

 to represent the septa of part of a corallite having 

 the formula S. 48, each septum being indicated 

 by the numeral proper to its order. 



The same numerals also enable us readily to 

 point out any one of the loculi included between 

 the primary septa. In the present case the 

 chambers, from left to right, would be denoted 

 as 1+4, 4 + 3, and so on, respectively. Again, 

 chambers of equal size are said to be similar, and 

 those represented by corresponding numerals are 

 of the same expression. Thus, in the diagram, 

 the chambers 1+4 and 4+1 have the same ex- 

 pression. So, also, just before the development 

 of septa of the fifth order, the chambers then 

 separated from one another by the septa marked 3 

 could not have been denominated similar. 



