ON THE BRITISH SPBOIES OF OEISIA. 



141 



the aculeata type, and never occurs in any other type of 

 colony. 



The method of branching and the number of zooecia in an 

 internode are far less constant than in the last two species. 

 Some of the characters of C. aculeata may, however, be 

 illustrated by a formula, using the letter s to indicate the 

 position of a spine, in addition to the symbols which have been 

 employed in other cases. 



Rootlet 





'=(1+^) 

 l=(6+,r+a;) 



'=(2+a;) 



The formula represents the whole of a main stem, given off 

 from a rootlet, together with the whole of one of its lateral 

 branches and a portion of another. In several important respects 

 this formula differs from those which have been given of the pre- 

 ceding species. It is by no means uncommon to find completely 

 formed internodes which have an even number of zooecia ; and 

 these even-numbered internodes do not conform to the denti- 

 culata or eburnea type by being usually without branches. 

 In some cases, indeed, an even-numbered internode has no 

 branch ; in other cases it has two branches, one on each side ; 

 in others again it may have one branch, or it may have a spine, 

 which, as has often been pointed out, is a structure which may 

 be regarded as a suppressed branch : like the true branches, 

 the spine has horny joints at intervals, and is attached to the 



VOL. V, PAET II. 



14 



