282 ERYLUS SOLLASII. 



difficulty in counting the rays of the large few-rayed acanthtylasters, but it is 

 iini)o8sil)le to ascertain the ray-numbers of the small many-rayed ones with 

 sufficient exactitude. I therefore calculated the means of the two- to nine- 

 rayed acanthtylasters by themselves, but combined the ten- to fourteen-rayed 

 ones in one group of which I took the mean. This mean can be taken as the 

 mean diameter of the asters with a ray-iiumbcr equivalent to the mean of 10, 11, 

 12, 13, and 14, that is twelve. In this way I found that the average diameter 

 of the acanthtylasters with two rays is 38 /£, 



of those with three " 29 ", 



" " " four " 28 ", 



" " " five " 27 ", 



" " " six " 25 ", 



" " " seven " 23 ", 



" " " eight " 21 ", 



" " " nine " 19 ", and 



" " " ten to fourteen (mean twelve) rays 14 /(. 

 Apart from the two-rayed asters, which are so few that I was unable to 

 measure a number sufficient for attaining a reliable mean, the mean given 

 above shows that there is a very regular decrease in size with increasing ray- 

 number, amounting in the asters with from three to five rays to 1 /x and the 

 asters with six or more rays to about 2 /( per unit of difference of ray-number. 



In all the forms four- to ten-rayed acanthtylasters have been observed. 

 The four- to six-rayed appearing to be the most frequent ones. In the forms 

 A, C, and D of race I and in race II also three-rayed and in race II also a few 

 two-rayed acanthtylasters were found. Acanthtylasters with more than ten 

 rays have lieen found in all the forms excej)t race I, form C. In tlic small speci- 

 mens the acanthtylasters are not smaller than in the large ones. The largest 

 acanthtylasters occur in the form A of race I and in the races II and III. (See 

 table, p. 283.) 



The aspidasters. The disc-shaped spicules of the cortical armour of the 

 species of Erylus have hitherto been designated, like the ovoid spicules of tlie 

 armour of Geodia, as sterrasters. Closer examination of these si)icules in the 

 species of Erylus of the "Albatross" collection has shown, however, that they 

 differ from the sterrasters found in the species of Geodia and allied genera not 

 only in their shape, but also in their mode of development, to such an extent 

 that it is advisable to give them another name. Aspidaster, the name selected 

 for them, has reference to their shield (do-Trt's)- like shape. 



