398 D'ARCY W. THOMPSON, HYDROIDA. 
but in I. plumosa they are more tubular, narrower, and the 
orifice points directly upwards rather than outwards. The dif- 
ferences in the gonangia are considerable, and are clearly 
shown in the figures. 
I am now inclined to think that the specimens which I 
referred to S. argentea in my report on Hydroids of the Willem 
Barents Expedition 1881, were wrongly referred to that species: : 
at least they represent a very distinct variety. The hydrothecee 
are more sharply bent and everted, in this respect exaggerating 
the features in which S&S. argentea differs from &S. cupressina ; and 
they were also larger, measuring about 0,43 or 0,44 mm., while 
those of British specimens of S. argentea measure only 0,34. 
The following measurements give the length of the hydrothece 
in a series of species of Thuiaria. 
Thuiorio: (SS) CUPPeSSTNG sd rk AAA ES AE Ö,34 mm. 
TD (0S-) AV EVO ba a EEE sek Er NEG. Sen NG AR EE ARN AE EES 
T--Öargeniea (var: from Willem Barents colecnemnit Öva ve 
PM ODUSCA Fr medd gas NR AA ANSE SA OSKET EES RESER NN 0,49 > 
TNG SEG EDET Ira BE 0 NEO AS ra OA 1 TE RANE 0 
I BESS ATK [ELR fö SNRA SR RR AR KASS AS ET ASEA SEN Ad os Eon ocooes 0,50 > 
2 BEE (EES Aer nr AR a An RN rs dö sön dss Ös2 
