274 
times being round without teeth and often being curved, with two 
teeth of regular sertularian type. Operculum usually composed of 
one flap attacbed to the abcauline side of margin, but sometimes 
composed of two flaps.” “This species appears to break down the 
generic distinctions proposed by Levinsen in that it has both 
a one-flapped and a two-flapped operculum in the same specimens. 
About the same parts of S., robusta he says: “operculum with two 
flaps on distal portion of branches, often with round margin and 
single abcauline flap on proximal portions.” 
Both in my paper on the regeneration of the Hydroids 1 ) and 
in that on the Hydroids from Greenland 2 ) I have pointed out, that 
iu the new apertures produced by the regeneration of a hydrotheca 
in a Sertularia the contrast between the thicker and the thinner 
(membranous) parts of the wall often seem to be indistinct or 
quite lacking, and as a distinet example hereof I have named 
Sertularia tenera. As I have examined many colonies of this 
species without tinding any other inconstancy in the parts named 
I am sure that the round apertures found by Pi of. Nu tt in g, 
must have belonged to regenerated hydrothecae and Ritchie 3 ) 
has come to the same result as I. Of Sertularia robusta I have 
examined a colony from Bering Sea sent to me by the National 
Museum of Washington. AU the hydrothecae present the Ser- 
tularia -characters very distinetly, and when Prof. Nutting in a 
number of hydrothecae from proximal portions of branches has 
found a different form of aperture and operculum, it is no doubt 
due to cases of regeneration. 
Nutting 4 ) declares that the operculum is almost an ideal 
character to use in separating the genus Diphasia, but that he 
nevertheless prefers the colonial characters is seen from his refei- 
ence of Sertularia thujarioides Clark to Thujaria, though it pos- 
x ) 82 a, p. 22. 
2 ) 82, p. 189—190. 
3 ) 58, p. 218. 
4 ) 44, p. 44. 
