396 Mr. W. J. Sollas on the 



XXXVII. — The Sponge-fauna of Norway ; a Report on the 

 Rev. A. M. Norman's Collection of Sponges from the Nor- 

 wegian Coast. By W. J. Sollas, M.A., F.R.S.E., 

 F.G.S., &c. 



[Continued from p. 259.J 



[Plate XVII.] 



Group Geodina, Carter (continued). 



Genus ISOPS *, nov. Type I. Phlegrcei, sp. nov. 

 (Geodia auct.) 



Diagnosis. — Excurrent and incurrent apertures similar, be- 

 ing the freely open ends of simple cylindrical tubes, which sink 

 directly 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 very 

 various sizes, forming a series in which the largest pass into 

 the smallest by almost insensible gradations. The larger 

 apertures serve as excurrent and the smaller as incurrent 

 ostia. 



The simple nature of the ostia and the identity in structure 

 of 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 as 

 equivalent to the chones of the preceding genera deprived of 

 their perforate (poriferous) roof. 



The excurrent ostial tubes differ from those of Geodia in 

 being simple and dispersed, the cribriform area or ostial tube 

 with many sphincters of Geodia being possibly due to the 

 integration or coalescence of a number of simple tubes 

 similar to those of Isops, thus : — 



1. Ostial tubes of Isops, closely congregated. 



2. Excurrent tube of Geodia, formed by the coalescence of similar 

 ostial tubes. 



la-os, equal ; $i\j/, an eye (hence a hole). 



