ON THE BRITISH SPECIES OP OEISIA. 113 



C. eburnea, Linn. Plate XI, fig. 6. 

 Zoarium forming dense tufts, usually attached by a single 

 stenij the base of which does not, in most cases, develop many 

 rootlets ; the average height of well-grown colonies from ^ to 

 f inch J the branches characteristically curved inwards. Inter- 

 nodes usually short, somewhat flattened; in most cases with 

 an odd number of zooecia, the dominant numbers being 5 and 

 7. Branches generally arising from the lowest zooecium in 

 an internode, sometimes higher up ; one branch is normally 

 developed from each odd-numbered internode, even-numbered 

 internodes being ordinarily branchless. On the main stem 

 or the principal branches, the branches come off in regular 

 alternation on opposite sides : nearer the growing-points, they 

 are arranged in compound helicoid cymes, of the formula^ — 

 (« + r,) + 



L („ + rO + 



1= (» + r,) + 

 &c. 



Joints yellow, or colourless near the growing-points, some- 

 times becoming dark brown in the older parts of the colony. 

 Basis rami short, not wedged in between two zooecia. Zooecia 

 almost entirely adnate, the upper portion, which bears the 

 aperture, free, bent forwards nearly at right angles to the lower 

 part; frequently a conspicuous pointed process on the outer 

 side of the aperture. O vie ell large, curved inwards, usually 

 replacing the second, or, less often, the third zooecium of an 

 internode; its aperture conspicuous, elongated from side to 

 side, borne on a very distinct tube, which is wider at its base 

 than at its summit. Rootlets usually developed' in very small 

 numbers. (See also measurements on p. 159.) 



0. eburnea. — 



(4) Johnston.— P. 283, pi. 1, figs. 3, 4. 



(5) Casus.— P. 38. 



1 This method of representing the branching is explained on p. 146. In 

 the above formula n would usually be 5, less often 7 or higher numbers. 



