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Show 410 PROF. F. J. BELL ON THE ECHINOMETRIDsE. [Mi 1. Observations on the Characters of the Echinoidea.-IV. The Echinometrida?; their Affinities and Systematic Position. B y P. J E F F R E Y B E L L , M . A . , F.Z.S., Professor of Comparative Anatomy in King's College, London. [Eeceived February 24, 1881.] In continuation of the observations which I have already had the honour of bringing before the Society, I enter on this occasion into an account of some of the characteristics of what is, perhaps, the most difficult group of all the Echinoidea. The remarkable, 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 Echinometra, 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 slighter than this universal consent would have inclined us to imagine. It is not necessary to recapitulate the history of the group ; the publication 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 Echinoidea look upon as the starting-point of their future labours. In the latest * Revision of the Echini,' the family " Echinome-tradae" 1 is accepted with very much the same kind of limitations as were suggested in 1855 by Dr. Gray2, who grouped his sixth family thus: - Fam. 6. E C H I N O M E T R A D S E. Ambulacral area only half as wide as the interambulacral area ; ambulacral pores in groups of four or more, forming an arched series round the ambulacral tubercles. A. Body circular. 1. Strongylocentrotus. B. Body oblong. 2. Echinometra. 3. Holo[i. e. Hetero~]centrotus. 4. Colobocentrotus. 1 Where Gray or Agassiz are quoted the term Echinometridee is spelt as they ^followed1 ° P a spelHng which' as J humbly imagine, is more correct, a P. Z. S. 1855, p. 37. |