128 SIDNEY F. HARMER. 



been reached iu the evolution of the genus. In his later works 

 he consistently refers to the latter ^' species^' as Crisia 

 eburnea, forma denticulataj and is of opinion that the 

 " species ^' recognised by other authors are^ for the most part, 

 merely partially fixed points in a continuous series. Most other 

 writers, on the contrary, regard these forms as so many distinct 

 species. 



In many of the characters used for distinguishing the several 

 species of Crisia from one another — such as the mode of 

 branching, the number of zooecia in an internode and their 

 individual shape, the position of the ovicells, &c. — each species 

 may vary within wide limits about a certain average. The 

 most satisfactory specific characters appear to me to be fur- 

 nished by the ovicells j and in this respect I am only confirming 

 the results previously arrived at by Waters^ for other Cyclo- 

 stomata. Indeed, I believe that in many cases the species 

 cannot be certainly identified unless ovicells are present. 

 Unfortunately, in the great majority of works referring to 

 Crisia, the information given with regard to these structures 

 is of the most unsatisfactory character. Many writers, for 

 instance, have mentioned the existence of ''pear-shaped'^ 

 ovicells in certain species; but this character is of generic 

 much more than of specific importance, and the same remark 

 might be made with regard to many of the other characters 

 which have been ascribed to the difi'erent species. 



The importance of the form of the ajaerture of the ovicell, 

 as a specific character, has almost entirely escaped the notice 

 of previous writers.^ Busk^ has merely stated that the existence 

 of a tubular aperture on the ovicell is a generic character of 

 Crisia. 



' '•' Ovicells of Cyclostomatous Bryozoa," ' Linn. Soc. Jouru. Zool.,' 

 vol. XX, p. 275, and in otiier places. 



^ Waters has, however, called attention to tlie importance of tliis cliaracter 

 in several vForks. See ' Quart. Journ. Geol. Soc.,' vol. xl (Nov., 1884), 

 p. 676. 



^ "Report on the Polyzoa," Second Part, '"Challenger" Hep., Zool.,' 

 vol. xvii, part 50, p. 2. 



