174 



GEODIA MICROPORA. 



centrum, 4-6 [x, about a quarter to a third of the whole aster, in diameter, from 

 which from sixteen to twenty-two concentric, regularly distributed rays arise 

 radially. The rays are straight, conical, sharp pointed, without the centrum 

 6-7.5 /< long and, at the base, 1-1.6 p. thick. The rays bear small spines. The 

 latter are sometimes so minute as to be hardly discernible as such. Often some- 

 what larger spines form a verticillate belt some distance below the end of the ray. 

 The total diameter of the oxysphaerasters is 14-20 p.. 



The sjnall strongylosphaerasters (Plate 36, figs. 18a, 19a, 20, 24b, 26b, 28, 

 33a, 34a) have a centrum 2-3 p, a third to a half of the whole aster, in diameter, 

 from which from eight to fifteen, most frequently twelve, rays arise. These are 

 generally regularly arranged, concentric, radial, and equal, rarely irregular. 

 The rays are cylindrical or cylindroconical, truncate, and often provided with a 

 terminal spine. They are 2-3 /j. long, at the base 0.6-1.3 /i thick, and bear rather 

 large spines, some of which often form a verticil some distance below the end of 

 the ray. The total diameter of the small strongylosphaerasters is 6-9.2 /j., usually 

 6-8 n. 



Besides these I have observed similar but smaller strongylosphaerasters, 

 down to 3.4 p in total diameter, which I take to be young forms of the ones 

 described above. 



The sterrasters (Plate 36, figs. 21-23, 30, 31, 35, 36) are broad, flattened ellip- 

 soids 72-76 fi, rarely as much as 82 p. long, 65-74 p broad, and 55-62 p thick. The 

 average proportion of length to breadth to thickness is 100 : 95 : 85. In the 

 centre of the sterraster there is a cluster, 2.5-5 p in diameter, of small granules. 

 The umbihcus is a conic pit, 11-15 p deep. Seen in profile the sides of this pit 

 appear somewhat concave. Its entrance is usually irregular in shape, often 

 considerably elongated, and measures 11-18 p in maximum diameter. The 

 freely protruding rays which surround the umbilicus have a transverse section 

 elongated in a direction radial to the centre of the umbilicus, which measures 

 2-2.5 p X 4: p. These rays bear as many as 8-12 lateral and several terminal 

 spines. The other protruding rays, away from the umbilicus, have more or less 

 circular transverse sections, which are 2.5-3.4 p in diameter. These rays 

 usually bear from 5-7 lateral spines. In sterrasters not quite fully developed 

 the spines are slender (Plate 36, figs. 35, 36), in the fully developed ones they 

 are stout (Plate 36, figs. 30, 31). 



This sponge was collected on April 13, 1888, at Duncan Island, Galapagos. 

 It was labeled F. C. 1354 and 539 Tetractinellida. 



The structure of the skeleton and the cribriporal character of the afferents 



