40 



SIDONOPS OXYASTRA. 



Geodia, which latter I have, for the reasons given in the description of Geodia 

 agassizii, also compared, at all resemble these sponges in their spiculation. 

 The only species similar to S. angulata is S. hicolor described in this Memoir, 

 and from this it is distinguished by the possession of anaclades and angularly 

 bent amphioxes, the smoothness of the oxyasters and oxysphaerasters, and 

 the smaller size of the sterrasters. 



Sidonops oxyastra, sp. nov. (^^A". '^?'^0<^ — Q) 

 Plate 6. figs. 1-23; Plate 7, figs. 1-20; Plate 8, figs. 1-15. 



I establish this species for two specimens from Duncan Island, Galapagos. 



The asters of this species are all oxyasters (oxysphaerasters) and to this the 

 specific name refers. 



The larger of the two specimens (Plate 6, fig. 5) forms a mass 94 mm. in 

 maximum diameter, attached to a flat, water-worn pebble, half of which it has 

 overgrown. Deep incisions partly divide this mass into lobes; the central 

 undivided part is 76 mm. broad; the lobes (Plate 6, fig. 4) taper distally and are 

 rounded. The surface appears undulating, smooth, and, to the unaided eye, 

 destitute of a spicule-fur. Large parts of the strongly convex and most exposed 

 portions of the surface are altogether without pores. In other exposed parts 

 a few minute pores are observed. The flat and the concave, more sheltered 

 parts of the surface are perforated by very numerous pores, two kinds of which 

 can be distinguished. The whole of the extensive fiat surface of the central 

 mass and considerable parts of the surface of the lobes are occupied by sieves 

 containing small afferent pores. In some places, where the dermal membrane 

 forming these pore-sieves has been rubbed off the entrances to the radial afferent 

 cortical canals are exposed to view. There is a tract 4-14 mm. in extent 

 occupied by a group of large and conspicuous efferent pores (Plate 6, fig. 4) on 

 nearly every lobe. 



The smaller specimen, which measures 56 mm. in maximum diameter, 

 resembles the larger one, described above, in every respect. It also grew, as 

 the impression in the detached base shows, on a flat pebble perhaps another 

 part of the one to which the larger specimen is attached. 



A monaxonid sponge (Plate 6, figs. 19a, 20a; Plate 7, figs, lb, 2b, 6b) and 

 composite ascidians incrust parts of the afferent areas of both specimens. 



The colour of the surface of the large undivided central mass is in both 

 specimens (in spirit) purplish brown, that of the other parts much lighter, 

 brownish white. The interior is Hght brown. 



