12 BROOKS—NEW GENUS HYDROID JELLY-FISHES. {April 4, 
and open into the circular canal by sixteen, thirty-two, or more 
distal branches; with two sorts of tentacles—hollow ones and solid 
ones ; with a simple mouth and with a single circumferential gonad 
which extends from the wall of the manubrium on to the radial 
canals and their branches. 
Dichotomia cannoides (Plate I, Figs. 1, 2 and 3). 
Diagnosis of the Species —Bell subcylindrical, somewhat higher 
than wide, with a conical apex. Manubrium fusiform, widest at 
about the middle of its upper half. The four radial canals branch 
dichotomously four (or more?) times. Near the apex the four 
primary canals arise in two pairs from the ends of a short trans- 
verse canal. There are sixteen long hollow tentacles, and about 
thirty-two (or more?) short solid tentacles. The reproductive 
organ extends from the wall of the manubrium on to the radial 
canals and their branches for about half their length. 
Special Description.—The four radial canals do not arise inde- 
pendently and directly from the aboral end of the stomach, but in 
pairs from the ends of a short transverse canal, in such a way that 
the only planes which divide the jelly-fish into symmetrical halves 
are the two primary interradial planes. When it is divided in 
either of these planes each half is itself bilaterally symmetrical, 
consisting of halves which are reversed copies of each other. In 
all my larger specimens each of the primary radial canals was 
divided dichotomously three times, so that there were eight sec- 
ondary canals, sixteen tertiary and thirty-two terminal branches. 
In one specimen, which is shown in Fig. 1, one of these terminal 
canals was again divided into two, so that there were thirty-three 
instead of thirty-two terminal branches. It is therefore probable 
that the number of branches continues to increase with the age of 
the jelly-fish, and that older specimens may have sixty-four or more 
terminal branches. The subumbrella consists of two strongly 
contrasted regions: an upper opaque portion which is nearly hemi- 
spherical and which contains the arches formed by the reproductive 
organ on the arched subdivisions of the radial canals, and a lower 
portion which is cylindrical and transparent. About one-half of 
the total length of the system of canals is joined to the reproduc- 
tive organ, which extends from the wall of the manubrium to the 
radial canals in a system of groined arches, dividing the upper 
part of the subumbrella into pockets which are closed above, open 
