250 



MR. F. J. BELL ON THE ECHINOIDEA. 



[Mar. 4, 



acquaintance with his specimens ; for he writes : — " The species of 

 this section are most difficult to distinguish ; they present several 

 variations, which at first sight appear characters, . . . but these varia- 

 tions do not appear to be permanent in the specimens of the same 

 habitat, but this fact requires verification with a larger series ; the 

 form of the fasciole is often different on the two sides of the same speci- 

 men" (p. 52). 



It is this statement of Dr. Gray's that gives a more accurate 

 account of the real facts of the case, though he might have added, 

 indeed, that the reentering angles vary greatly in depth. Of fifteen 

 specimens which I have examined from the large series in the Museum, 

 four have one reentering angle on either side in the anterior inter- 

 ambulacra ; one has no angle on the left, and one on the right side ; 

 one has one angle on the left and a shallow one on the right side ; 

 four have one angle on the left and two on the right side ; and five 

 have two on both sides. With the series in my hands I am there- 

 fore unable to come to any conclusion from Agassiz's second dis- 

 tinctive character. 



With regard to the other point, the relative lengths of the anterior 

 and posterior ambulacra, I have first to say that in no case that I 

 know of are the anterior longer than the posterior ambulacra ; and 

 among such cases I reckon the representation given by Prof. Agassiz 

 (pi. xxi. fig. 1) ; and, secondly, that of nine specimens selected, 

 that in which the cariuate character of the posterior odd interambula- 

 crum was least well marked, had anterior ambulacra measuring 40 

 millims., and the posterior 43 millims., while in that in which the 

 carination was most marked the anterior ambulacra measured 38 

 millims., and the posterior 40 millims. 



The following Table gives some details as to the just-mentioned nine 

 specimens, which are arranged in an increasing order of carination, 

 as judged by the eye, and are all apparently well-grown specimens, 

 since all are more than 100 millims. in length : — 



Table I. 



In the next Table is given the proportions of five specimens from 

 one locahty, Naples ; and it will fitly lead to the series of smaller 

 forms. 



