ERYLUS ROTUNDUS. 

 DIAMETERS OF SMALL OXYASTERS. 



305 



The aspidasters (Plate 5, figs. 27f, 28f ; Plate 6, figs. 17, 18; Plate 7, figs. 16- 

 30, 42-45; Plate 8, figs. 13, 14) are I:)n)ad-oval or circular discs, often with a 

 somewhat irregular outline. This irregularity of outline generally does not 

 exceed that of the aspidasters represented in figs. 18, 19, and 25 on Plate?; 

 occasionally, however, quite ii'regular aspidasters, with one or more deep inci- 

 sions reaching far into the interior, have been observed in all varieties. The 

 aspidasters are 50-77 /« long, 46-70 /« broad, and 4.4-8.8 /c thick. Those of 

 var. megarhabda. (50-66 by 46-59 by 4.6-8.8 /<) are smaller than those of the 

 other two varieties. The general average proportion of length to breadth to 

 thickness of the aspidasters of all the forms (varieties) is 100: 93.3: 10.2. The 

 asj)idaster-disc is either of uniform thickness throughout, or slightly thickened 

 in the middle. Its margin is simj^ly roimdcd off. An umbilicus could not be 

 detected. All parts of the surface, the margin as well as the two faces, are 

 covered with protuberances. The largest protuberances are 0.7-2 /< thick and 

 about as high. Most of them bear a terminal verticil of usually from three to 

 seven exceedingly small lateral sjMnes. The large protuberances are usually 

 scattered rather irregularly over the surface. Occasionally some of the pro- 

 tuberances of the central part of the aspidaster lie in straight lines, radiating 

 from the centre of the disc. Between the larger protuberances small ones, just 

 perceptible with the strongest lenses, lie singly or in small groups on the other- 

 wise smooth surface of the disc. 



