4 JO PROF. F. J. BELL ON THE ECHINOMETRTD.E. [Mar. 15, 



1 . Observations on the Characters of the Echiuoiclea. — IV. 

 The Echinometrida; their Affinities and Systematic 

 Position. By F. Jeffrey Bell, M. A., F.Z.S., Professor 

 of Comparative Anatomy in King's College, London. 



[Received February 24, 1881.] 



In continuation of the observations which I have already had 

 the honour of bringing before the Society, I enter on this oc- 

 casion into an account of some of the characteristics of what is, 

 perhaps, the most difficult group of all the Echinoidea. The remark- 

 able, though only apparent, asymmetry of the test of some of the 

 Echinometridae can only receive its rational explanation from the 

 results of developmental studies ; it is not, however, idle to prepare 

 for these by giving some definite information as to the parts and 

 proportions of the constituent tests. 



The genus Ecliinometra, with the asymmetrical forms allied 

 thereto, Heterocentrotus and Colobocentrotus, have, by the almost 

 universal consent of naturalists, been closely associated one with 

 another ; and there is as yet no evidence which would justify us in 

 offering any real opposition to these views. On the other hand, 

 when we come to investigate the kind, and to weigh the amount and 

 value, of the characters which have led to the union just mentioned, 

 we find them to be shghter than this universal consent would have 

 inclined us to imagine. 



It is not necessary to recapitulate the history of the group ; the 

 pubHcation of a Revision should save us from that, where we feel 

 enabled to follow it ; and I purpose, therefore, to begin with what 

 students of the Echiuoidea look upon as the starting-point of their 

 future labours. 



In the latest ' Revision of the Echini,' the family " Echinome- 

 tradEe"'is accepted with very much the same kind of hmitations 

 as were suggested in 1855 by Dr. Gray', who grouped his sixth 

 family thus: — 



Fam. 6. Echinometrad.e. 



Ambulacral area only half as wide as the interanibulacral area ; 

 ambulacral pores in groups of four or more, forming an arclied series 

 round the ambulacral tubercles. 



A. Body circular. 



1. Strongyloceyitrotiis. 



B. Body oblong. 



2. Echinometra . 



3. Holo[i. e. Hetero]centrotus . 



4. Colobocentrotus. 



' Wliere Gray or Agassiz are quoted the term Eckinometrida is spelt as they 

 spelt it ; in other places a spelling which, as I humblv imaxrine, is more correct 

 is followed. " ' 



" P. Z. S. 1855, p. 37. 



