ON THE BRITISH SPECIES OP OEISIA. 109 



On the British Species of Oris ia. 



By 



Sidney F. Harmer, M.A., B.Sc, 



Fellow and Lecturer of King's College, Cambridge. 



With Plate XL 



This paper will be followed by a further memoir, which will 

 treat of the development of the ovicells and of the embryos in 

 Crisia. I have already published a preliminary note^ on this 

 subject, and I hope to be able to complete the preparation of 

 the more detailed paper without much delay. 



It has often been pointed out that the subdivision of the 

 Cyclostomatous Polyzoa into genera and species is attended 

 with peculiar difficulties. The character of the zooecia remains 

 remarkably constant throughout this group, the systematic 

 study of which is not facilitated by the presence of subsidiary 

 structures, such as the opercula, avicularia, and vibracula, 

 which in the Cheilostomata form so valuable a means of 

 distinguishing the species. 



The task of finding satisfactory specific characters within the 

 limits of the genus Crisia is not less difficult than in other 

 genera of Cyclostomata, as is seen clearly enough by 

 examining the numerous works which have already been 

 devoted to this genus. Smitt, for instance, in his critical 

 analysis of the Scandinavian forms, has asserted that the 

 delicate C. geniculata is connected by a continuous series of 

 intermediate forms with the coarse form which he calls C. denti- 

 culata, and which he regards as the extreme point which has 

 1 'Proc. Cambridge Philosoph. Soc.,' vol. vii, part 2, 1890, p. 48. 



VOL. V, PART II. 12 



