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Show 418 PROF. F. J. BELL ON THE ECHINOMETRIDsE. [Mar. 15, Turning our attention now to the Echinidfe, we may define them as Regular Echinoidea, with external gills and five pairs of ambulacral plates on the buccal membrane, in which some sets of primary plates always fuse to form a secondary ambulacral plate, in which the auricular arch is complete, and the rudimentary internal gill entirely lost. If the above be, then, a good and fair definition of the Echinidse, we come to a consideration of the points by which its constituent genera may become grouped into distinct subfamilies. The tables already given show that it is hopeless to expect to be able to find any ground of distinction on the absolute number of pairs of pores in an arc ; we cannot say that, at any one point, forms with three pairs of pores end and those with four begin. The character, not being a constant or absolute one, is unfitted for use as a family-character ; nor are there any points which we can propose as affording so wide a distinction between Echinometra and Echinus. Personal observation can only confirm the general tendency of the researches of Perrier, Stewart, and Mackintosh on the histological characters of the group in question, and lead to acquiescence in the conclusion of M. Perrier:-" On le voit, les modifications qui caracterisent les Echinometriens sont parfaitement nettes, mais ce ne sont que des modifications dans le type des Echiniens. Le type ne change pas comme lorsqu'on passe du Cidaris aux Diademes, et de ceux-ci aux Echinocidaris ou aux Oursins proprement dits." These considerations appear to me to be sufficient to justify us in retaining Echinometra, Strongylocentrotus, Echinus, andl'oxopneustes in one family, and to refuse to follow Dr. Gray or Prof. Agassiz in forming a family Echinometradse as distinguished from the Echinidse. If we look yet a little further we shall find that the elaborateness of the ambulacral plates, the strength of the spines, the size of the buccal apparatus, appear to have culminated in Heterocentrotus and ColoBocentrotus rather than in Tripneustes and Toxopneustes, which in the latest Revision are, in the systematic list, placed furthest from the Cidaridse. Whatever be the significance of the obliquity of the morphological axis, there can be but little doubt that it is of very great importance ; and a return to the definition of " body circular," and to the recognition of the differences insisted on by Johannes Miiller, seems to be better than a vague union of forms, elevated into a family for no better reason than one that has already (p. 413) been quoted and discussed. A scheme, therefore, of the following character will probably throw into prominence the points of likeness and unlikeness in the constituent members of the family Echinidse. Group I. Body circular ECHININSE. (a) Secondary plates formed of three primary plates e. g. Echinus. (ft) Secondary plate formed in adult |