ON THE BRITISH SPECIES OF OBISIA. 155 



C. cornutEj its zooecia being distinctly longer and thinner than 

 in that species; and spines seem to be never developed. As 

 already remarked, however, parts of the colonies of C. cor nut a 

 may be devoid of spines. 



Breeding Period and Occurrence of Species. 



The specimens from which the following statements are made 

 have been received at various periods from February to the end 

 of August. I have to offer my best thanks to those who have 

 most kindly assisted me by supplying me with material ; and 

 especially to the staff of the Marine Biological Association, 

 Prof. H. de Lacaze-Duthiers, and Mr. J. Sinel. 



The dominant species at Plymouth is certainly C. ramosa; 

 although, strangely enough, I have not been able to obtain this 

 form from any other place, except from a bottle found in the 

 Morphological Laboratory at Cambridge. The contents of 

 this bottle came either from the Channel Islands or from Arran ! 



At Plymouth, C. ramosa is found commonly at depths from 

 4 to 30 fathoms. It is particularly fond of growing on stones, 

 but is found on other objects — e. g. glass bottles, shells, red 

 seaweeds, Cellaria, and sponges. When it grows in the last 

 position it appears to be in danger of being killed by the sponge, 

 which grows over its branches. The Crisia is, however, gene- 

 rally able to keep pace with the growth of the sponge, so that 

 only the basal parts of its colonies are killed, or at least prevented 

 from having functional polypides by the sponge. The speci- 

 mens growing at 4 — 6 fathoms were usually much more luxu- 

 riantly branched than the few specimens which I received 

 from 20 — 30 fathoms ; and ovicells were obtained only from 

 those growing under the first set of conditions. 



C. eburnea is also common at Plymouth, but is almost 

 always found on red seaweeds or on Sertularia. The restric- 

 tion of various species of Crisia to particular seaweeds, &c., 

 has been often noted by previous observers. 



Winther^ has stated that, in the Danish forms of this species, 



1 G. Winther, " Fortegnelse over de i Danmark hidtil fundne Hav-Bryozoer," 

 ' Naturh. Tidsskrift ' (Kj^benhavn), 3 Raekke, vol. xi, 1877-8, p. 7. 



