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Show 41 MR. F. J. BELL ON A NEW GENUS OF ECHINOIDS. [Feb. 3, was not mentioned or described by the late Dr. Gray in his 'Cata-lo gue of the Recent Echinida,' published in 1855. It is much worn and has much the appearance of a fossil specimen ; in the opinions ot Prof. Morris and of Dr. Henry Woodward, F.R.S., however, the specimen is recent. It was bought at Stevens's sale-rooms, and is, with a doubt, reported to have come from India. To the accomplished palaeontologist just named I have to express m y thanks for instructed guidance through the cabinets under his charge ; and while I take on myself all responsibility in describing this form, I would say at once that it is the fact that it was unknown to Dr. Woodward which has chiefly led me to regard it as new. As we pass in review the edentulous forms of the Irregular Echinoidea, we are led irresistibly to the conclusion that the shortening of the ambulacra and the arrangement of the pores in the mode which has led Prof. Hackel to give to the group the name of Petalosticha1, are structural changes which have gone hand in hand ; and, just as we may say of the ungulate Mammalia that their limbs tend to become modified by the reduction of the outer digits, and that, where success is attained, this reduction is accompanied by concomitant changes in the relations of the metacarpal or metatarsal bones to the carpus or tarsus2, or of the Araneina that they have tended to limit their stigmata to two3, so may we say of the Petalosticha that the arrangement of the ambulacral pores in straight parallel rows is more ancient than that in which the greater number are set in petaloid fashion. So far the generalization is borne out by the evidence of the palseontological succession, while some of the observations of Alex. Agassiz seem to support it on the embryological side. Perhaps we may go a step further and say, with safety, that the longer, the more regular, and the straighter are, step for step, older arrangements than rows of pores less long, less regular, or less straight. It is obvious that all kinds of stages may be found in this series if the regular and orderly modification of the Echinoderm structure has taken place in the non-saltatory fashion which it is now the mode to ascribe to the process of Evolution; but there is another possible process which it is, after all, not so much more difficult to present distinctly to the imagination; and that is progression by leaps of varying breadth4. Prof. Agassiz has drawn attention to the sudden transitions which he has observed in the growth of an individual, and to the apparently sudden appearance of genera in their geological succession. Let us test these two conflicting views by the evidence afforded by the new genus; but before doing so, let us point out that, even if we shall find evidence in favour of the sudden or, as we may call it, saltatory character of the transitions, it is just what we seem to find also in the developmental history of the individual; so that it affords us, just as well as any more steady succession, quite as complete a demonstration of the 1 Gener. Morphologie, ii. p. lxxiv. 2 Kowalevsky, Proc. Roy. Soc. 1873, p. 153. 3 Cf. Bertkau, Archiv fiir Naturg. 1878, pp. 351 et seq. 4 In the sense, of course, that the intermediate forms were so rapidly passed over that the chance of their being preserved is practically nil. |