Limitation of Genera among the Hydroida. 855 
Hydractinide. 
1. HypractinrA, Van Beneden. 
Trophosome. — Ccenosare forming a continuous expansion 
whose free surface is destitute of periderm, but whose deeper 
parts consist of an areolar mass of freely intercommunicating 
tubes, which are each invested by a distinct periderm, and are 
adnate to one another. Polypites claviform, developed at inter- 
vals from the free naked surface of the ccenosarc; tentacles fili- 
form, approximated into a single verticil round the base of a 
very contractile and mutable metastome. 
Gonosome.—Sporosacs supported on gonoblastidia, which 
arise, like the polypites, from the naked free surface of the cc- 
nosare, are destitute of tentacles, and terminated by spherical 
clusters of thread-cells. 
Hydractinia echinata, Flem. (sp.), ¢ ,= Hydractinia lactea, Van 
Ben., 2 ,= Hydractinia rosea, Van Ben., = Synhydra parasites, 
Quatref., = ?Dysmorphosa conchicola, Philippi. 
Hydractinea polyclina, Agass. 
2. Rurzociine*, Allman, nov. gen. 
Trophosome.—Ccenosare forming an adherent stratum sup- 
ported by “a solid chitinous expansion” +. Polypites developed 
at intervals from the free surface of the coenosarc; tentacles 
filiform, in a single verticil round the base of a conical meta- 
stome. 
Gonosome.—Gonophores phanerocodonie, sessile on the free 
surface of the ccenosarc. Umbrella, at the time of liberation, 
deep bell-shaped; manubrium large, with a four-lipped mouth, 
but not extending beyond the margin of the umbrella; four 
radiating canals continued distally by four marginal tentacles 
with bulbous bases; three shorter tentacles developed in each 
interradial space. 
Rhizocline areolata, Alder (sp.),= Hydractinia areolata, Alder. 
Laride. 
1. Lar, Gosse. 
Trophosome.—Ccenosarc a creeping, filiform, and anastomosing 
hydrorhiza, on which sessile polypites, are developed at intervals ; 
* From pia, a root, and kdivn, a bed. 
+ Mr. Alder describes the attached base of the Hydroid for which I have 
found it necessary to constitute the present genus as “ consisting of a solid 
chitinous expansion, from which arise simple linear spines in regular groups 
having areolar spaces between them.’ There can be no doubt, however, 
that, with specimens favourable for observation, he would have discovered 
a fleshy coenosare in connexion with the chitinous basis. 
