14  BROOKS—NEW GENUS HYDROID JELLY FISHES, _ [April 4, 
der Medusen, p. 158), although it is so different from the Williadz 
and in fact from all the ‘‘ Leptomedusz ’’ that it may turn out to 
be a tubularian jelly-fish, or ‘‘ Anthomedusa.’”’ The simple manu- 
brium and mouth, the hollow tentacles and the origin of the gonad 
in the wall of the manubrium are all points of agreement with the 
‘© Anthomedusz,’’ The solid tentacles have an axis made up of a 
single row of chorda cells, but as tentacles of this sort are found in 
undoubted tubularian jelly-fishes they afford no ground for exclud- 
ing the genus Dichotomia from this group. 
Prof. Walcott has described, from the Lower Cambrian of Ala- 
bama, certain remarkable fossils (Fossil Medusa, by Charles 
Doolittle Walcott: Monographs of the United States Geological 
Survey, xxx, Washington, 1898) which he regards as the remains 
of Medusz, and it is worthy of note that if the Medusa which is 
here described were slightly distorted by pressure the digestive and 
reproductive organs would exhibit some resemblance to one of the 
surfaces of some of Walcott’s most characteristic types. At his 
suggestion I made a model in clay of the reproductive organs of 
Dichotomia in order to exhibit this resemblance, and Fig. 3 was 
drawn from this model. The resemblance lends additional support 
to the opinion that the Cambrian fossils are the remains of Medu- 
se, although it does not indicate that there is any relationship 
between Dichotomia and the fossils. In fact the resemblance is 
only superficial. In all the general details of their structure the 
Cambrian Medusze must have been very different from the one that 
is here described. 
The notes and drawings for this paper were made in 1888, 
although I have been forced to delay their publication. 
EXPLANATION OF PLATE I. 
Fic. 1, An adult specimen of Dichotomia cannoides, enlarged about six diame- 
ters. From a drawing made at Nassau, New Providence, in 1886. 
Fic. 2. A young specimen, enlarged about twenty diameters. From a drawing 
made by R. P. Bigelow at Bimini, Bahama Islands, in 1887. 
Fic, 3. A clay model of the reproductive organs of Fig. 1. 
