ON THE BRITISH SPECIES OE CRISIA. 149 



true of actively growing colonies found early in the year : at a 

 later period, when growth is less energetic, the apertures may be 

 much less prolonged, and in many cases they are not more 

 prominent than in some specimens of C. denticulata, even 

 in the case of those zooecia which are not closed by a diaphragm. 

 Lower in the stem the tubular mouths are, in most cases, lost ; 

 the zooecium is closed by an oblique diaphragm, and no poly- 

 pide is present. The internode is then a flattened structure, 

 in which the apertures of the zooecia project even less than in 

 C. denticulata. 



In an interesting abnormality found in August, two promi- 

 nent tubular apertures occurred, side by side ; the extra 

 zooecium being in the position which would normally have been 

 occupied by a basis rami. 



Busk^ has made rather a point of the fact that in C. conferta 

 the free tubular portion of the zooecium is not a mere produc- 

 tion of the peristome, but presents " the same puncturation as 

 is seen on the rest of the cell." This is certainly the case in 

 C. ramosa and in C. aculeata, and to a less extent in C. 

 denticulata. C. eburnea is, in fact, the only species I have 

 examined in which the tubular portion is usually merely a thin 

 prolongation of the peristome. 



The average position of the ovicell is somewhat higher than 

 in C. aculeata; but that it varies greatly in position is ob- 

 vious from the formula — 



{i+0v. + 19+r,+„r+,r+r,) + 



It has never been noted to be lower than 4th in the internode ; 

 but it is seldom so low as this. The branch may be jointed 

 above the ovicell, although most commonly the ovicell is borne 

 by a terminal internode, which usually possesses at least two 

 branches. "When several branches are developed, two of them 

 are usually developed not far above the ovicell, one on each 

 side of the internode, whilst the other branches are developed 

 from the lower parts of the internode. This is the case in 

 ' ' Catalogue .... Brit. Museum,' part iii, p. 7. 



