254 
Sertxdarella by the strictlv alternate position of its hydrothecae, 
H i n c k s lays the chief stress on the structnre of the latter which 
are pro vided with 3—4 marginal teeth and with a similar number 
of opercular valves. While he has found no operculum in Sertu - 
laria and Hydrallmania . and in one of the two Thujaria- species 
named in his work, he characterizes Diphasia by the presence of 
an inner (adcauline) operculum, but as he has not seen that a 
quite similar operculum is also present in Abietinaria abietina and 
Ab. jilicula he refers them to Sertularia. In the diagnosis of the 
different genera the author also uses the form of the colony and 
the structure of the gonothecae, but he expresslv emphasizes that 
the whole arrangement must only be regarded as provisional: L ) 
“Without the examination of a much larger number of foreign 
species, the genera of this family cannot be defined with certainty 
and precision; and the present grouping must be accepted as to 
some extent, provisional”. 
Professor G. J. All man, who has published so many valuable 
works on the Hydroids, in opposition to Hincks has great con- 
fidence in the systematic importance of the colonial characters, 
and in one of his papers 2 ) he even institutes a new family Thu- 
jariidae , solely based on the mode of division of the hydrocaulus 
into internodes. In his report on the Hydroids from the Challenger 
expedition he unites Sertularella with Sertularia as he has found 
an operculum in some species of the latter genus, and evidently 
cannot imagine that a generic difference might be expressed through 
the different form and position of something so delicate and perish- 
able as the operculum, about the systematic significance of which 
he says :i ): “The valves in all these cases are so thin and perish- 
able that it is only in recent or exceptionally well preserved spe- 
cimens we can hope to meet with them, a faet which in itself 
deprives the distinetions, derived from them of that practical value 
M 22, pag. 260. 
2 ) 2, pag. 267. 
3 ) 5, pag. 51. 
