. The Annals and magazine of natural history; zoology, botany, and geology. of simple cylindrical tubes, which sinkdirectly into the rind of the sponge and end at its inner sur-face in sphinctral muscles. Observations.—The ostia of the canal-system are of veryvarious sizes, forming a series in which the largest pass intothe smallest by almost insensible gradations. The largerapertures serve as excurrent and the smaller as incurrentostia. The simple nature of the ostia and the identity in structureof the incurrent and excurrent ostia and ostial tubes con-stitute a good distinction between this
. The Annals and magazine of natural history; zoology, botany, and geology. of simple cylindrical tubes, which sinkdirectly into the rind of the sponge and end at its inner sur-face in sphinctral muscles. Observations.—The ostia of the canal-system are of veryvarious sizes, forming a series in which the largest pass intothe smallest by almost insensible gradations. The largerapertures serve as excurrent and the smaller as incurrentostia. The simple nature of the ostia and the identity in structureof the incurrent and excurrent ostia and ostial tubes con-stitute a good distinction between this genus and its allies—Geodia, Cydonium, and Pachymatisma. The incurrent ostial tubes of Isops may be regarded asequivalent to the chones of the preceding genera deprived oftheir perforate (poriferous) roof. The excurrent ostial tubes differ from those of Geodia inbeing simple and dispersed, the cribriform area or ostial tubewith many sphincters of Geodia being possibly due to theintegration or coalescence of a number of simple tubessimilar to those of Isops, thus :—. 1. Ostial tubes of Isops, closely congregated. 2. Excurrent tube of Geodia, formed by the coalescence of similarostial tubes. la-os, equal; $i\j/, an eye (hence a hole). Sponge-fauna of Norway. 397 How far other distinctive characters will be found con-stantly associated with those given as diagnostic of the genusIsops, one cannot at present say. As yet only a single repre-sentative of the genus (I. Phlegrcei) is known to me; betweenit and Geodia Barretti, however, very characteristic differencesexist. Thus the former does not possess any of the smallacerate spicules, which, projecting erectly from its surface,form the fine superficial down of G. Barretti; they appear tobe entirely replaced by the large long-shafted spicules whichform the externally projecting fascicular ends of the internalspicular fibres. The trifid heads of the long-shafted spicules,moreover, do not, as in G. Barretti, apply themselves with
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