258 
decussating pairs, but Ritchie?) has enlarged it to also include 
species with more than four longitudinal series of hydrothecae, 
characterizing it by the arrangement of the hydrothecae in alternate 
transverse series, each containing two or three hydrothecae; but 
a somewhat similar arrangement is also found in a number of 
Selaginopsis-species and according to. Broch?) we not rarely find 
in Sel. mirabilis four longitudinal series of… hydrothecae in the 
place of six, and in that case the hydrothecae are, as in Stauro- 
theca dichotoma, arranged in decussating pairs. 
Even if all the species referred to Selaginopsis possessed a 
special form of operculum not found in any other genus, I should 
only regard the polyserial arrangement of the hydrothecae as a 
systematic character of lower rank, but.it is evident from the 
above enumeration of species that there is not.the slightest con- 
nection between this arrangement and the form of the operculum, 
and the former is as a rule not even constant within the colony, 
as in most species the hydrothecae are biserial in the stem, and 
besides, there has been found a number of species belonging to 
Diphasia, Thujaria and Sertularia, which present both biserial and 
triserial hydrothecae either in the same or in different branches, 
and sometimes in different colonies.  Diphasia Wandeli, in the 
original specimen of which all the branches have three series of 
hydrothecae, is nearly related to D. fallax, and is perhaps only 
to be regarded as a variety of the latter species. Sæmundsson?”) 
refers to D. Wandeli four colonies, three of which have only bi- 
serial hydrothecae while in the fourth, which possesses 17 pairs 
of branches, two at the one side and six at the other have only 
biserial hydrothecae in their proximal portion and triserial in the 
rest of their length. Broch") has described a Thujaria, to which 
he has given no name as he is not sure of its identity. In the 
1) 50, p. 538. 
2?) 13, p. 173. 
Sy Så, p. 97. 
38 np 191. 
