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Show 18H0.] MR. F. J. BELL ON A NEW GENUS OF ECHINOIDS. 49 from Echinoneus it is distinguished by the fact that the paired pores do not extend from apex to actinostome ; with the remainder of the Cassidulids, however, it has considerable affinity. In several points it presents resemblances to Rhynchopygus; but this curious genus is sharply distinguished by the position of its anus ; Neolampas has but a double row of simple pores * in each ambulacral area; Echinolampas has one of its two paired rows of pores considerably shorter than the other in each ambulacrum; and Conoclypeus, with which Palceolampas presents the greatest affinity, differs from it in having retained the primitive position of the mouth, and in nevertheless having obtained large bourrelets, as well as having the pairs of pores united by grooves2; Conoclypeus also has the apical system specially modified into a projecting button-like piece, and has the ocular plates very small. On the whole, then, Palceolampas seems still to retain in its organization points of structure which show that it either diverged from the Cassidulid stem rather earlier than Conoclypeus, or that it stands in the direct line which connects this genus, first seen in the Cretaceous epoch, with a still more generalized ancestor. As it presents, therefore, a grade in development, we can only justly recognize the value of the characters which it presents by forming for it a special genus; and the name which is proposed seems to be one that it is justified by the characteristics herein detailed. Returning to the question with which we started, we find, I think, that the existence of an intermediate form of this kind, continued on for so long a period of the world's history, as it is almost certain it has been, must make us very careful as to accepting any statements which seem to throw discredit on that principle of most modern evolutionists, which ascribes the origin of species to the effects of variations, not always seen by the unobservant eye. And while the explanation suggested as to the instability of certain combinations of anatomical characters may throw light on some of our difficulties, it is hardly yet time for us to cease giving the proper weight to our limited opportunities, and the imperfections of the records of the past, or, on the other hand, to forget how species best adapted for investigation are not always those that have most completely retained an uneffaced record of their past changes. DESCRIPTION OF PLATE IV. Fig. 1. Palceolampas crassa: view of the abactinal surface, to show the disposition of the rows of pores, and the character of the plates of the corona. Natural size. 2. P. crassa : view of actinal surface, to show the characters of the bourrelets and phyllodes in the distribution of the ambulacral pores. Nat. size. 3. P. crassa : profile view. Nat. size. 4. Abactinal system, enlarged, to show the size of the ocular and genital plates, the characters of the ocular pores, and the position of the madreporic plate. 1 See W y ville Thomson, ' Phil. Trans.' vol. clxiv. pt. 2, p. 745. 2 In C. leskii the actinostome is some way in front of the centre; but it is a question whether this species truly belongs to the genus Conoclypeus. PROC. ZOOL. Soc-1880, No. IV. 4 |