132 



GEODIA AGASSIZII. 



TOTAL DIAMETER AND DIAMETER OF THE CENTRUM OF THE SMALL STRONGYLO- 

 SPH.\ERASTERS OF GEODIA AGASSIZII. 



The nonyial stemisters (Plate 27, figs. 15-19; Plate 30, figs. 11-17; Plate 

 31, figs. 1, 2, 5-7; Plate 32, figs. 32, 35; Plate 33, figs. 1-8, 12, 13) are abun- 

 dant in all specimens. The full-grown sterrasters of the adult specimens are 

 flattened ellii)soids 82-118 /i long, 75-100 /i broad, and 58-83 /( thick, their 

 average maximum dimensions being 103.55 by 88.56 by 69.22 /«. The largest 

 are those of the specimen from Station 3088. The proportion of length to 

 breadth to thickness is fairly constant. Those of the specimen from Station 

 3088 are relatively somewhat longer and those of the specimens from Stations 

 2886 and 4228 relatively somewhat thinner than those of the (tthers. In the 

 young specimen from Station 4228 the full-grown sterrasters are similar in 

 shape, 76-100 /i long, 70-85 /« broad, and 60-70 ii thick, their average maximum 

 dimensions being 95 by 83 by 68 /«. In the immature specimen described by 

 Lambe the sterrasters are 90-110 by 74-92 by 67-75 /<. (See table p. 133.) 



On one of the two broader sides of the normal full-grown sterrasters an 

 umbilicus, usually more or less circular in outline, 12-15 fi in transverse diameter, 

 and 6 ft deep, is observed (Plate 27, figs. 15-19; Plate 31, figs. 1, 2, 5-7). With 

 the exception of a small, smooth, central patch at its bottom, the wall of the 

 umbilical pit appears to be roughened (Plate 31, figs. 5, 7). I am not quite 

 positive, however, whether there really is a roughness there, it being quite 

 po.ssible that its appearance in this place may be an optical illusion, caused by a 

 refraction at the surface of the umbilical pit, that in fact this apparent roughness 

 is in reahty nothing but a blurred ultraviolet light-image of the rays and spines 

 on the other side of the surface, which are traversed by the light before it reaches 

 the umbilicus. Observations with high powers in ordinary light failed to decide 

 this question. 



From the whole of the surface of these normal, full-grown sterrasters, with 

 the exception of the part occupied by the umbilicus, the distal ends of the rays 



