CAITLOPHACELLA TENUIS. 65 



General structure of the skeleton. Both faces of the lamella are covered 

 with pinules occupying the usual position. The pinnies on one side are much 

 larger than those on the other. The face covered with the larger pinules I 

 consider the dermal, the opposite face the gastral. Pentactines, with para- 

 tangentially extending lateral rays, and an apical ray directed radially inwards, 

 occur under both surfaces. The pentactines underneath the face with the larger 

 pinules are much larger than those underneath the opposite face. The former 

 are considered as hypodermal, the latter as hypogastral. Numerous slender 

 and some large rhabds, a few large hexactines, and dense masses of oxyhexasters 

 and hemioxyhexasters occur in the interior (Plate 12, fig. 13). The oxyhexasters 

 are much more numerous than the hemioxyhexasters. 



Small hexactines with rays strongly curved at the end ; large sword-shaped 

 hexactines with stout and spiny sword- handle ray; middle sized hexactines with 

 cylindrical, terminally rounded, strongly curved raj^s; and a few other forms 

 have also been observed in the spicule-preparations. I consider these spicules 

 as foreign to the sponge. 



The rhabds are 3-17 mm. long, 10-34 n thick, rarely 50 m, and quite sharply 

 pointed. 



The rare large hexactines have straight, conic rays, 0.5-1 mm. long and 

 20-45 M thick at the base. The rays are smooth for the greater part of their 

 length, their tips only being covered with small tubercles. 



The hypodermal and hypogastral pentactines differ only in regard to their 

 size. Their lateral rays are straight, conic, and rather sharply pointed. Those 

 of the former are 500-1100 ^ long and 14-32 m thick at the base; the correspond- 

 ing measurements of the latter are 110-330 n and 7-14 ix. 



The dermal and gastral pinules (Plate 12, figs. 1-8, 14, 15, 19) also differ only 

 in size. In both the distal ray is straight, stout at the base, and only slightly 

 thickened above. Sparse, small spines arise from its basal part. Towards 

 the end the spines become more crowded and larger, the largest attaining a 

 length of 7 M in the dermal pinules and a length of 5 /i in the gastral. These 

 spines are sharp-pointed, directed obliquely upward towards the tip of the ray, 

 and also curved in this direction. Their basal part is inclined at an angle of 

 about 60° to the ray ; farther on they bend, usually somewhat abruptly, towards 

 the ray ; so that the angle between their end-part and the ray is 45° or less. 

 Usually the spines of the same region are fairly uniform. Sometimes, however, 

 adjacent spines differ considerably in position. Occasionally I have observed 

 pinules in which the distal spines all tended to one side as if bent by a lateral 



