Limitation of Genera among the Hydroida. 377 



a creeping filiform hydrorhiza; hydrothecse with an operculum 

 formed of converging lanceolate segments. Polypites cylindri- 

 cal, with a verticil of filiform tentacula connected at their basea 

 by a broad membranous web. 



Gonosome. — Gouophores phanerocodonic. Medusje 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^quorea vitrina, Gosse, become 

 developed into polypoid trophosomes having the characters enume- 

 rated above. The Mquorea vitrina of Gosse, however, is not a true 

 JSquorea, 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 {%^.) = jEquorea vitrina, Gosse. 



Thaumantidse. 

 1. Thaumantias, Eschscholtz. 



Trophosome. — Ilydrocaulus rooted by a creeping filifcJrm hy- 

 drorhiza ; hydrothecse 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 Thaumantias is here defined in accordance with the hmits 

 assigned to it by Gegenbaur, who restricts it to such Medusae as possess the 

 characters given above. Thaumantias as thus Umited 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 

 Medusae." 



Ann. ^ Mag. N. Hist. Ser. 3. Vol. xiii. 25 



