774 



MR. A. G. THACKER ON 



[June 16, 



The canal-system is typically leuconoid ; the flagellated chambers 

 are spherical and have a diameter of from 0*03 to 0*05 mm. 

 Many of the incurrent and excurrent canals are very wide ; they 

 attain a diameter of 1 mm. and in some cases of even more. 



The Skeleton consists of the following elements : — (1) Large 

 triradiates of the parenchyma. (2) Triradiates of the dermal 

 cortex. (3) Sagittal quadriradiates and triradiates of the gastral 

 cortex. (4) Large oxeotes, only slightly thicker than the large 

 triradiates. (5) Minute oxea. (6) Minute spined grapnel-like 

 oxea of the gastral cortex ; these latter form the main distin- 

 guishing character of the species. Finally, the two smaller speci- 

 mens have oscular fringes consisting of long fine smooth oxea. 



The large triradiates of the parenchyma (text-fig. 164, a), 

 which make up the main mass of the skeleton, are more constant 

 in shape than those in Polejaefi^s .specimens. They are regular 

 or very nearly regular and have sharp-pointed rays. The average 

 size, from which there is not much variation, is — length 0*45 mm., 

 basal width 0*045 mm. A small minority of the spicules depart 

 from the regular form and are sagittal or irregular in shape, both 

 rays and angles being unequal. 



Text-fig. 164. 



Spicules from Leucandra rudifera, 



a -= Parencliymal triradiates X 24. S = Denial triracliate X 24. c = Gastral 

 quadriradiate X 24. d = Large oxeotes X 24. e = Small oxeotes (of dermal 

 cortex etc.) X 120. f = Minute spined grapnel-like oxeotes of gastral cortex 

 X500. 



The triradiates of the dermal cortex are variable in shape, being 

 either sagittal or irregular ; the thickness of their rays is about 

 half that of those of the parenchymal triradiates, being on the 

 average 0-022 mm. ; the rays are frequently slightly curved, are 

 fairly sharply pointed, and reach a length of 0*35 mm. (text- 

 fig. 164, 6). 



The gastral cortex consists almost entirely of quadriradiates 



