On new Species of Hydractinildae. 9 



have apparently been washed about in the surf for years before 

 they were picked up for preservation? Meanwhile, as the 

 description of a polypidom alone is comparatively easy, as it 

 may be a long while before the soft parts can be obtained, and 

 as it is desirable at once to separate these skeletons from the 

 sponge-structm-es which I am examining, it is hoped that 

 the following diagnoses may not be unacceptable. 



Hydractinia levispina, n. sp. (PL I. fig. 1, a, b.) 



Zoophyte incrusting and eroding univalve shells. Poly- 

 pidom formed of clathrate, subrectangularly meshed chitinous 

 fibre (as in fig. 9), solid, concentrically laminated, surmounted 

 by smooth, erect, conical spines (figs. 3 & 1, Z», e), grouped 

 together in the midst of proliferous tubercles (fig. 1, d,e), 

 scattered more or less over the surface. Increasing by layers, 

 so as finally to absorb the whole of the shell on which it grows 

 (fig. 2, a, b) . Height of transformed shell 2 inches from apex 

 to base ; extreme breadth, viz. from left side to margin of outer 

 lip, 1^ inch. Spine variable, about l-30thinch high by l-60th 

 inch diameter at the base. 



Hab. Unknown. 



Loc. Unknown. 



Obs. This specimen, which is in the British Museum, bears 

 the number "2461," which mode of marking, as before stated, 

 shows that it has been there for a very long time ; the number 

 also appears to be preceded by a "P." There is no further 

 history attached to it than that which its own structure reveals. 

 It evidently grew on a shell a little less in size than a Buccinumj 

 but of a totally different species, as the margin of the aperture 

 is continuous like that of the Turbinida3. While there it 

 gradually transformed the whole of the shell into its own 

 chitinous polypidom ; meanwhile a Pagurus or hermit crab 

 inhabited the interior and so preserved the form of this part. 

 Subsequently it probably got into some tidal estuary, where, 

 having been left high and diy on its banks, a gymnospermous 

 lichen took up its habitation on its surface, and, spreading its 

 thallus throughout the external layer of the imperishable 

 chitine, at last threw up the groups of shield-like conccptacles 

 (apothccia) now scattered over the greater part of the siiell-like 

 polypidom. Of course this might also have taken place on the 

 sea-shore, or the Piujurus itself might have carried it inland. 



Hydractinia levispina differs from H. echinata in the 

 tubercled state of its surface, but especially in the smoothness 

 of its spines (fig. 3); the latter possesses a more or less even 

 surface with serrated spines (fig. 4). 



