of Shell-substance hy Hydractinia. 5 



The transformed shell bears the museum no. " 2461," which 

 appears to me to be jj/eceded by a P ; and the former shows 

 that it must have been in the museum for many years, since 

 for upwards of twenty this kind of numbering has been dis- 

 continued. 



In its maximum measurements it is about two inches long 

 from apex to base, two inches broad, i. e. from left to right 

 side, and one inch thick. 



The outer layer is rendered more or less green by the 

 presence of the gonidia and thallus of a lichen, which here and 

 there appears in little groups of gymnocarpous apothecia all over 

 the surface. So it is just possible that, after the comparatively 

 heavy calcareous matter of the shell had been replaced by the 

 lighter chitinous structure, the Pagurus^ as before stated, left 

 his habitation ; and the latter, having floated into an estuary, 

 may have been left on its banks, where its surface became in time 

 grown over by this lichen, aitd where, probably, it was found, 

 unless all this took place on the sea-shore, or the Pagurus carried 

 the transformed shell inland, as they appear to do in the island 

 of Cuba (Sir C. Lyell, Princip. Geol. vol. ii. 1872). 



The largest apothecia are about l-48th of an inch in diameter, 

 and more or less circular, the thalamiura dark brown, and ex- 

 ciple white ; the spores ellipsoid, generally eight in the theca, 

 but varying in number, and for the most part confusedly 

 arranged. 



My attention was first called to the specimen of Ilydractinia 

 ecliinata above mentioned from its likeness to the figures of 

 the sponge named " Teiyios echinata''' by De Fonbressin et 

 Michelotti (' Spongiaires de la Mer Caraibe,' p. 102, pi. xxiv. 

 figs. 4 & 5, Haarlem, 1864). And then, when I observed 

 coupled with it in the museum another shell like it, but entirely 

 transformed into horny structure, I began to think that the 

 skeleton of Hydractinia ecliinata must be a sponge, not being 

 aware at the time that any organism but a sponge could effect 

 such a transformation, and observing microscopically that the 

 horny substance was formed of concentric layers. However, 

 placing the specimens before my friend Mr. Parfitt for his 

 opinion as to tlie habitat and species of the lichen, this intel- 

 ligent naturalist immediately recognized Hydractinia ecMnata^ 

 and handed out from his cabinet a specimen dredged up off the 

 Otter-mouth, close to the place where I am living. The nature 

 of the organism on the whelk-shell thus having become known 

 to me, that of the organism which had transformed the other 

 shell still remained enigmatical, but was subsequently worked 

 out in the way above mentioned. 



It would appear from a section of the crust that the poly- 



