412 Prof. F. J. Bell on Amphiura Chiajli, Forbes. 



longer than those of older specimens ; thus the length of the 

 arms was about five times the diameter of the disk in a speci- 

 men whose disk-diameter was 5 millim., and four times that 

 of the disk when the disk-diameter was 8 millim. ; on the 

 other hand, a specimen whose disk-diameter was 9 millim. 

 had an arm nearly 70 millim. long, but this, which was the 

 only comj)lete arm on the specimen ^ was for the greater part 

 restored. 



The process of restoration, indeed, may be observed in 

 almost every specimen, and an examination of one will enable 

 us to estimate the value of the character, which, in his "Key" 

 to the species of Amphiura^, Mr. Lyman gives as a distin- 

 guishing characteristic of A, Chiajii — " a notch in outer side 

 of under arm- plates." 



If we take an arm which, by the darker colour of its proxi- 

 mal and the lighter colour of its distal portion, indicates that 

 it is a ray which has undergone repair, we shall find a marked 

 difference between the under arm-plates of the old and the new 

 joints. The former will be seen to have three sides very 

 nearly even, as even, at any rate, as plates that were notched 

 at an earlier stage may be expected to have them ; but the 

 latter will be found to be constricted from side to side and to 

 be more or less emarginulated along their distal edge. 



The extent to which these notches at the edges of the plates 

 become filled up depends first of all on age, and secondly, no 

 doubt, on the amount of carbonate of lime which the individual 

 Amjphiura is able to appropriate to the strengthening of its 

 delicate skeleton. 



Forbes's remark that the lower plates are longitudinally 

 sulcated is, so far as my experience goes, a character that 

 is so constant that it should not be omitted in any definition 

 of the genus. 



The comparative length of the arras and the extent of 

 calcification of the under arm-plates are not the only points in 

 which examples differ. Specimens collected at one spot and 

 preserved in the same manner differ in the form of the disk, 

 which may be nearly circular or may be more or less deeply 

 incised or angulated in the interradial areas. The extent to 

 which the apical and the central plates are obvious is another 

 point in which, as a rule, young differ from old specimens ; 

 as may be supposed, it is in the former that these significant 

 plates are best marked. Another character which presents 

 diflferencesj and generally^ though not always, differences with 

 age, is the general coloration of the body ; if we take a speci- 



* ' Challenger ' Eeport on Ophiurids, p. 123. 



