•32 PLATE XII. 



The character of the spinulation of the skeleton 

 spicula is best exhibited in those from the l^asal mass 

 of the sponge, and in those with a power of 108 hnear, 

 in consequence of the attenuation of the shaft towards 

 its basal portion, they are not strikingly demonstrated. 

 To obtain a satisfactory view of them they should be 

 seen with a power of about 200 linear. The spinulate 

 character in the skeletons of the cloacas are rarely 

 visible. The spinulation of the spiculum is not purely 

 so, and the enormi-spinulate form is of most frequent 

 occurrence. Sometimes the spinulation is very close 

 to the basal end of the spiculum, and at other times it 

 is at two or three times the diameter of the spiculum 

 from the base, and occasionally, but rarely, there is a 

 second inflation on the shaft of the spiculum eight or 

 ten diameters above the primary one, as rej^resented by 

 fig. 8, pi. xi. In young and immature skeleton spicula 

 the spinulation is more strikingly displayed than in the 

 adult one, as represented by tig. 11, X 123. As the 

 spicula arrive at maturity the inflation becomes lessened 

 in its proportions as compared with the diameter of the 

 shaft, as in figs. 5 and 0. 



Polymastia mammillaris is found in Larne Lough 

 attached to stones at low-water mark, and can only 

 be obtained at unusually low spring tides. The 

 specimen figured was obtained by Mr. W. Darragh, 

 March 25th, 1849. When alive and in the water he 

 says it is of a j^ellow colour. 



