Limitation of Genera among the Hydroida. 355 



HydractinidflB. 

 1. Hydractinia, Van Beneden. 



TVophosome. — Coenosarc 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 coenosarc ; 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 coe- 

 nosarc, are destitute of tentacles, and terminated by spherical 

 clusters of thread-cells. 



Hydr actinia echinata, Flem. (sip.), 6 , = Hydractinia lactea, Van 

 Ben., $,= Hydradinia rosea, Van Ben., = Synhydra parasites, 

 Quatref., = Wysmorphosa conchicola, Philippi. 



Hydradinea polyclina, Agass. 



3. Rhizocline*, Allman, nov. gen. 



Trophosome. — Coenosarc forming an adherent stratum sup- 

 ported by " a solid chitinous expansionist- Polypites developed 

 at intervals from the free surface of the coenosarc; tentacles 

 filiform, in a single verticil round the base of a conical meta- 

 Btome. 



Gonosome. — Gonophores phanerocodonic, sessile on the free 

 surface of the coenosarc. 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. 



Rhizodine areolata, Alder {s^.),= Hydradinia areolata, Alder. 



Laridse. 

 1. Lar, Gosse. 

 Trophosome. — Coenosarc a creeping, filiform, and anastomosing 

 hydrorhiza, on which sessile polypites are developed at intervals ; 



* From piCa> « root, and kXi'm;, a bed, 



t Mr. Alder describes the attached base of the Hydroid for which 1 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 favom'able for observation, he would have discovered 

 a fleshy cceuosarc in connexion with the chitinous basis. 



