Limitation of Genera among the Hydroida. 377 
a creeping filiform hydrorhiza; hydrothece with an operculum 
- formed of converging lanceolate segments. Polypites cylindri- 
cal, with a verticil of filiform tentacula connected at their bases 
by a broad membranous web, 
Gonosome.—Gonophores phanerocodonic. Meduse with a 
broad, shallow umbrella and wide and short manubrium whose 
lips are prolonged into’ numerous arm-like lobes; radiating 
canals very numerous; marginal tentacles very numerous, radial 
and interradial, each developed from a bulbous base; margin of 
umbrella with lithocysts; generative elements developed along 
the course of the radiating canals. 
We are indebted to Dr. T. S. Wright for the important observation 
by which he has shown that the ova of A’quorea vitrina, Gosse, become 
developed into polypoid trophosomes having the characters enume- 
rated above. The Zquorea vitrina of Gosse, however, is not a true 
Aquorea, but, as Agassiz has pointed out, belongs rather to Zygodac- 
tyla of Brandt; and Zygodactyla vitrina thus becomes one of the 
few Hydroids in which the development of the trophosome from the 
ova of the free Medusa has been observed. The characters of the 
trophosome here given are those presented by this part of the Hy- 
droid in the most advanced state to which it had been traced by 
Dr. Wright ; but it must not be lost sight of, that it probably un- 
dergoes further changes before arriving at its completely developed 
condition. 
Zygodactyla vitrina, Gosse (sp.) = Aiquorea vitrina, Gosse. 
Thaumantide. 
1, THaumantiAs, Eschscholtz. 
Trophosome.—Hydrocaulus rooted by a creeping filiform hy- 
drorhiza ; hydrothecz destitute of an operculum. Polypites with 
the tentacular verticil surrounding the base of a trumpet-shaped 
metastome, 
Gonosome.—Gonophores phanerocodonic. Mature Medusa 
with a hemispherical umbrella and short manubrium with lobed 
mouth-margin ; radiating canals four; marginal tentacles nu- 
merous ; lithocysts absent; generative sacs band-like along the 
course of the radiating canals *. 
We are indebted to an observation of Dr. T. S. Wright for our 
knowledge of the trophosome of Thaumantias, Dr. Wright having 
seen developed from the ova of Thaumantias inconspicua, Forbes, a 
* The genus Thawmantias is here defined in accordance with the limits 
assigned to it by Gegenbaur, who restricts it to such Medusz as possess the 
characters given above. Thawmantias as thus limited will include the T. 
inconspicua of Forbes, so far as it is possible to judge from the description 
of this Medusa given in the “ Monograph of the British Naked-eyed 
Meduse.” 
Ann. & Mag. N. Hist. Ser. 3. Vol. xin. 25 
