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Show 1881.] PROF. F.J. BELL ON THE ECHINOMETRIDsE. 411 As subgenera of Strongylocentrotus, Prof. Alex. Agassiz includes Sphcerechinus and PseudoBoletia, the former of which Dr. Gray would appear to have included with Echinus in his fifth family, while the latter is a genus of which no species was then known. Echinostrephus had not, in 1855, been distinguished from Echinus or Psammechinus; while Stomopneustes, under the title of Helioci-daris, was also regarded by Gray as closely allied to Echinus. Of the nine genera, or subgenera, found in the family of the Echinometradse of Agassiz, viz. (I) ColoBocentrotus, (2) Heterocentrotus, (3) Echinometra, (4) Parasalenia, (5) Stomopneustes, (6) Strongylocentrotus, (7) Sphcer-echinus, (8) PseudoBoletia, (9) Echinostrephus, the first three and the sixth alone fall into Gray's family, the fourth and the eighth were unknown to science, while a different view was taken as to the affinities of Stomopneustes, Spha?rechinus, and Echinostrephus. They were regarded, in fine, as being more closely allied to Echinus, because they have the "ambulacral area half as wide as the interambulacral area, with two (or three) close series of double pores, placed in threes ; buccal membrane naked; body circular." W e may dismiss the first character, without even examination ; for, while it is obviously artificial, it is the same for Gray's two groups of Echinidse and Echinometradae. As to the second difference, the arrangement of the pores, there can be little doubt that, judging by it only, Stomopneustes has a much closer affinity to the Echino-metridee than to the Echinidse. And we now come to what is really the kernel of the whole matter. How far is Desor's division into Oligopori and Polypori natural? and how far is it artificial? If we examine one of the least modified of the Echinidse, e. g. Cidaris triBuloides, we find that the pores of the ambulacral zones are arranged regularly and equally in pairs, are, in effect, set one behind another in a straight line, and belong each to a single simple plate. If we take a more modified form, such as a species of the restricted genus Echinus, we find the pairs of pores have, for the greater part of the test, come to be set in arcs of three ; and on close examination it is seen that the plates connected with these pairs of pores are not all of the same size, and that the primary plates fuse to form a secondary plate1. This is the typical arrangement among the Oligopori; but it by no means holds for all the plates ; those nearest the apical area have, more or fewer, the pairs of pores in just as straight lines as Cidaris triBuloides. Taking, as an example of the Polypori, Echinometra suBangu-laris, we have some six pairs of pores arranged in a much more elaborate arc, and the changes that come to be effected are so great that what form really the distal pair of pores of one arc seem to be the proximal pair of the succeeding arc. 1 It seems to me that all the advantage lies in continuing to use the nomenclature of Johannes Midler, and to speak of the first or simple plates as primary, and the fused plates as secondary ; for reasons which, no doubt, are excellent, Prof. Alex. Agassiz has (op. cit. pp. 642, 643) elected to reverse this nomenclature and to speak of the compound plate as the primary one. 27* |