Vou. 2] Torrey—Hydroids of the San Diego Region. 25 
esting that of three successive branches from a short section of 
the same stem (fig. 18), the origin of the first is typical of The- 
cocladium, the origin of the second and third is typical of Ser- 
tularella. Without denying the usefulness of the usual manner 
: 
“4 
Fig. 17.—S. halecina. A younger stem than Fig. 18.—S. halecina.—Stem with three 
is shown in fig. 18 with two extrathecal branches two of which are extrathecal, 
branches. the third intrathecal, in origin. The 
stem is old, with damaged hydrothecae. 
of branching, or any other typical character, in defining species, 
the facts which have just been enumerated hardly supply the 
idea of stability which is commonly associated with the concep- 
tion of a genus. 
