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Show 78 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jail. 28, structure as well as from its external aspect, in the preceding manner. The type specimen was a widely expanded cup 16| inches in diameter. It was divided into about equal parts ; one half remains in the Bristol Museum, and the other is in the British Museum ; the sides rather exceeded an inch in thickness. The expansile dermal system, which usually contains the most strikingly characteristic parts of such sponges, is entirely absent from the general mass of the animal. The nature of the dermal membrane, the pores, and the oscula are therefore unknown to us ; but without the aid of these organs there still remain sufficient permanent specific characters to enable us to readily separate this species from its nearly allied congeners, in their present denuded state. Of the two species in the British Museum, Dactylocalyx pumiceus and Iphiteon Ingalli, the latter has been figured by Dr. Gray in the 'Proceedings' of this Society for 1867 (plate 27. fig. 2), and has been erroneously designated Dactylocalyx pumicea; and this error is the more remarkable as the surface-characters of the two specimens differ very materially from each other. The outer surface of D. pumiceus is furnished with deep channel-like depressions, disposed in irregular lines radiating from the basal portion towards the margin of the sponge. These channels or large interstitial spaces penetrate deeply into its substance, so as to convey within it the newly imbibed streams from the inhalant pores. Ou the upper surface of the sponge these channels do not exist; but in lieu of them there are numerous large round or oval orifices, varying in diameter from about two lines to nearly half an inch. There is a slight tendency to an arrangement in lines radiating from the centre to the circumference. There can be little doubt of these orifices being the terminations of the great excurrent system of the sponge, and that above each of them in the living state there would be the true oscula of the dermal system of the sponge, /. Ingalli differs materially in its surface-characters from D. pumiceus. The inner surface of the cup is furnished with numerous deep channels or depressions with sharp margins, while in D. pumiceus the corresponding part of the sponge is occupied with numerous circular or oval orifices with rounded margins; the outer surface of /. Ingalli is furnished with deep more or less sinuous channels with rounded margins, while the similar channels in D. pumiceus are decidedly arranged in nearly straight lines. Beside these differences in external appearance, the characters of their respective skeletons at once separate them not only as species, but as genera. The irregular structure of D. pumiceus is readily to be distinguished from the characteristic symmetrical configuration of the circular confluent areas of Iphiteon. There is also in the British Museum a piece of D. pumiceus, about 2 inches long by 1^ inch broad and about \ inch thick, on a tablet, said to be from Barbadoes ; this is probably a fragment off the large specimen from the Bristol collection, as its microscopical characters agree precisely with those of the large portions which I''have examined. There is also a small specimen of the species in the Belfast Mu- |
