1879-3 MR. F. J. BELL ON THE ECHINOIDEA. 



Table II. 



251 



The differences in the length of the ambulacra observed in the last 

 specimen seem to point to this character, so far as it is one, being 

 variable with age. 



In the following Table four smaller specimens are compared, and 

 seem to support this supposition : — 



Table III. 



The first two of these specimens were undoubtedly regarded by Dr. 

 Gray as belonging to the species carinatus. 



From these two tables we may perhaps draw the following con- 

 clusions : — (1) In specimens of £i-issus longer than 100 millims., 

 the proportion of the anterior to posterior ambulacra may be from 

 i^u u to li"o~7^- (-) ^^ specimens less than 100 millims. long the same 

 parts may be to one another in the proportions from yy-°f to {!]g[] \ 



If, then, the characters above detailed are of no value or assistance 

 in the discrimination of the species, we are led to ask what importance 

 is to be attached to the possession of a keeled posterior interambula- 

 crum ? its absence in young forms, its variability in older ones, and 

 the littoral habitat of its possessors points rather to its being a charac- 

 ter acquired by the individual itself than by the individual from its 

 ancestors. In other words, the variations in carination seem to be such 

 as are compatible with the general characters of the species ; it is a 

 ready instance of the difficulty of discriminating between mere varia- 



^ Verrill's measurements of his species B, ohesus bear out this conclusion ; his 

 largest specimen measured 2 inches, the anterior ambulacra '65, and the posterior 

 ■80, giving thus a proportion of { " !] P. (and not, as b}' some curious sliii, Verrill 

 states of ft- [}g?f]). 



