110 



GEODIA MESOTRIAENA. 



varieties; their average breadth, however, is greater in var. megana and var. 

 pachana than in var. microana. The proportion of length to thickness is in the 

 sterrasters of all three varieties 100 : 70-71 ; the proportion of length to breadth, 

 on the other hand, in var. pachana and var. megana 100 : 90-91, in var. microana 

 100:79. 



The umbilicus is a round (Plate 25, figs. 2, 3, 9-11), or more (Plate 25, fig. 

 G) or less (Plate 25, fig. 7) elongated, cup-shaped depression or pit, 10-18 p. in 

 maximum diameter. On the walls of the umbilical pit low irregular elevations, 

 covered with numerous minute spines, often forming protruding tufts, are met 

 with (Plate 25, figs. 9, 10). These elevations appear as ridges, extending from 

 the rays which surround the umbilical pit down towards its bottom, or as isolated 

 patches, the transverse diameter of which is similar to that of the rays. The 

 lowest part, bottom, of the umbilical pit is usually quite free from spined pro- 

 tuberances. The whole of the sterraster-surface, with the exception of the 

 part occupied by the umbilicus, is covered with protruding, cylindrical rays, 

 circular in transverse section, and usually about 3 p. thick, which terminate 

 with a rather flat apical face. From the margin of the latter stout and blunt, 

 conical spines arise. The axes of these spines are more or less vertical 

 to the ray-axis, so that they appear as verticils round the summits of the rays 

 (Plate 25, figs. 6-8, 11). Away from the umbilicus the rays are mostly crowned 

 by regular verticils of five or six spines (Plate 25, fig. 8). The rays surrounding 

 the umbilicus are provided with a greater number of spines, sometimes with as 

 many as eleven (Plate 25, figs. 6, 7). From the ends of many of these perium- 

 bilical rays, not only the lateral spines forming the verticil, but also more or fewer 

 upright spines arise (Plate 25, fig. 6). The spines attain a length of about 1.3 // 

 and are, at the base, about 1 thick. They are conical, straight, or more or less 

 curved, and blunt. Those of the periumbilical rays are on the whole larger and 

 more strongly curved than those of the rays on other parts of the sterraster. 

 In a few sterrasters, one in a hundred or less, the rays are larger and distally 

 crowned with a greater number of spines. These abnormal sterrasters appear 

 as transitions between the regular sterrasters and the sterroids. 



The specimen of var. pachana was trawled at Station 2909 on January 8, 

 1889, in 34° 22' N., 120° 8' 30" W., depth 375 m. (205 f.); it grew on a bottom of 

 green mud; the bottom temperature was 7.3° (45.2° F.); that of var. microana 

 was trawled at Station 2942 on February 5, 1889, in 33° 38' 45" N., 118° 13' 45" 

 W., depth 37 m. (20 f.); it grew on a bottom of gray sand and broken shells; 

 the specimen of var. megana was caught with the tangles at Station 2958 on 



