136 A MONOGRAPH OF THE 



discrimination of the two species comparatively easy and 

 satisfactory. 



7. Microciona ambigua, Bowerbank. 



Sponge. Coating; surface even, slightly hispid. Oscula 

 simple, dispersed, minute. Pores inconspicuous. 

 Dermal membrane, spiculous ; spicula acerate, long 

 and slender ; sub-fasciculated. Skeleton. Columns 

 very short, and indistinctly produced ; spicula atte- 

 nuato-acuate, basally spined, large and long, irregular 

 in size. Internal and defensive spicula attenuato- 

 ac.uate, entirely spined ; short and stout. Interstitial 

 membranes ; tension spicula cylindrical, entirely 

 spined, frequently sub-attenuated, distorted, and curved 

 considerably ; very numerous. Retentive spicula 

 angulated bidentate equi-anchorate ; and, rarely, pal- 

 mated equi-anchorate. 



Colour. Wet condition, light ochreous yellow ; dried, 

 light brown. 



Habitat. Shetland. 



Examined. Fresh from salt-and- water pickle, and in 

 the dried state. 



The specimen described above coated the expanded 

 extremity of one of the valves of a large shell of Pinna 

 ingcns (?), It covers a space of seven and a half by six and 

 a half inches, and in no part exceeds a line in thickness. 

 On the smooth portions of the shell the surface of the 

 sponge is even, and in the wet condition no oscula could 

 be detected by the aid of a lens of an inch focus ; but in 

 the dried state several were apparent in the mature portions 

 of the sponge. In the young and progressing margins of 

 the sponge the extreme edge appears as a thin film of 

 gelatinoid matter, in which a few tension spicula are 

 irregularly dispersed; within this marginal portion the 



