408 Dr. W. G. Ridewood—A Key for the Ready 
words, the synopsis itself should be consulted by those wishing 
to discriminate between these species. As regards the two 
diminutive species of the subgenus Demiothecia, Harmer 
writes (‘ Pterobranchia of the ‘‘ Siboga ” Expedition of 1899- 
1900,’ Leiden, 1905, p. 4) :—“ The possibility is not excluded 
that C. siboge is the male form of C. gracilis.” 
A study of the large and varied collection of C. densus 
obtained by the ‘Terra Nova’ leads to the conclusion that 
what Andersson described as C. rarus is but an early colony 
of C. densus, with the tubes of the ccencecium lax, straggling, 
and irregular, instead of closely set and more or less parallel 
—see ‘Terra Nova’ Report; pp. 39-40. 
Gravier’s species—C. anderssoni—is with difficulty distin- 
guishable from C. densus; his description of the zooids is 
incomplete, and the principal feature that distinguishes the 
ccencecium of his species is the aggregation of the tubes into 
clumps or clusters which stand out more or less distinctly 
from the other clumps—see ‘Terra Nova’ Report, pp. 40 
and 76. 
The present key is so drawn up as to bring the species 
nigrescens and solidus together. Although belonging to 
different subgenera, they have many points in common, and 
I was for some time uncertain whether the cone-shaped 
colonies obtained.on the Australasian Antarctic Expedition 
of 1911-1914 were small, short-tubed colonies of C. solidus or 
unbranched colonies of C. nigrescens—see report on the 
Pterobranchia of the expedition, Sydney, 1918, pp. 19-20. 
The arms of well-preserved zooids of C. nigrescens show a 
characteristic double black band on the axis, but the bands 
are lost in badly preserved material. On the other hand, 
it is not definitely known that the zooids of C. solidus do not 
possess such bands ; Andersson does not mention them, and 
the zooids of one of his specimens that I had an opportunity 
of studying do not show them ; the material, however, is not 
well preserved, and there are evident signs of the colour of 
the zooids having become diffused and reduced in intensity. 
The key is also arranged so as to bring together the two 
specially arenaceous species agglutinans and evansi; thie 
former has black zooids and the latter white. Although 
C. agglutinans differs from the other species of /diothecia in 
the tubes not ending blindly in the middle of the branch, the 
character is not readily determined, owing to the transparency 
and thinness of the tubes and the confusing effect of the 
numerous particles of shell embedded in the ccencecial 
substance. 
