ON THE BRITISH SPECIES OF CETSIA. 135 



alone sufficient to distinguish C. denticulata from the other 

 British forms. They are situated at a very low level on the 

 zooecia which bear them, and each "basis rami" (Smitt) 

 appears to he wedged in between two consecutive zooecia (fig. 

 3), instead of being, as in other species, distinctly apposed to 

 the outer side of one zooecium (figs. 4, 6, 11). The branches 

 usually originate from z^} z^, or z^ ; less commonly from stj 

 or from z^. 



The joints, both of the zoarium and of the rootlets, of this 

 species are in nearly all cases of a jet-black colour, as recog- 

 nised by most of the previous writers.^ The young joints are, 

 as in other species, uncoloured ; but the black colour is in 

 almost all cases very speedily acquired. Smitt and Busk do 

 not mention this as a specific character, no doubt because they 

 have given wider limits to the species than are accepted by 

 most writers. 



The ovicelP in all species replaces an ordinary zooecium, 

 and in this particular species it is usually borne on a lateral 

 branch, and in most cases is situated at some distance above a 

 joint. In the instances given in the formula on p. 146 the ovi- 

 cell replaces the 4th, the 6th, or the 10th zooecium of an in- 

 ternode. I have never seen it lower than 4th nor higher 

 than 13th. It is usually very near the end of a branch, and 

 this feature is well shown in pi. iv, figs. 2 and 4, of Busk's 

 British Museum Catalogue (Part III). In one of my cases, 

 however, thirteen zooecia occurred above the ovicell, and eleven 

 below it, and very rarely a joint may be developed above it. 

 If the ovicell-bearing internode develops a branch, that branch 

 is very seldom given off from a position higher than the 

 zooecium which corresponds to the ovicell on the opposite side 

 of the branch. 



' I. e. from the third zooecium of either right or left side : the side from 

 which a branch comes off has no significance unless considered in relation to 

 other characters. 



- Cf. Fleming, J., 'Hist. Brit. An.,' p. 540; Johnston, A., 'British 

 Zoophytes,' 2nd ed., p. 284 ; Hincks, T., ' Brit. Mar, Polyzoa,' p. 423 ; &c. 



' See also p. 169, 



