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Show 412 PROF. F. J. BELL ON THE ECHINOMETRIDsE. [Mi Turning now to the mode by which these pores come to be so arranged in the adult, it will be well to recapitulate shortly the present state of our knowledge concerning it. This knowledge has, within the last decade, been very considerably advanced by the elaborate and beautiful researches of Prof. LoveV. I shall depart from a strict following of his account only in using the^term " secondary plate " as a translation of his " plaque composes" As has been already pointed out, these secondary plates, when developed, are made up of three or more primary plates. Now, " the primary ambulacral plates of the Echinidse are either entire (that is to say, they occupy the whole of the distance between the interradial area and the median suture of the ambulacrum2, or, in other words, extend from the interradial area as far as the middle of the entire plates), or they end by a more or less sharp point. The major primary plates of the peristome forming the series \a .. . V B, most often consist, in very young individuals, of a first entire primary, of a median primary half plate, and of a third entire primary plate." In an appended table the learned author shows the arrangement of the entire and half plates in the several secondary plates of the corona of a small specimen of Toxopneustes (Strongylocentrotus) droBachi-ensis. The fourth or fifth of these has two complete and three half primaries, as is shown by the formula-1, (2, 3, 4), 5. Next we come to the mode of growth of these different primary plates. "Near the aboral edge of a complete composite plate there is deposited the first primary plate of the new plate, then the second, and soon. All the primary plates, and even the half-plates, are primitively entire plates ; that is to say, they extend from the interradial area as far as the median suture of the ambulacrum3. Later on, and during the period in which the entire collection of primary plates constituting the composite plate goes on enlarging, and even before it is completed by the last primary plate, the intermediate plates cease to grow ; and while retaining their position on the edge of the ambulacrum, beside the interradial area, they shrink at their extremities, which become separated from the median suture. They consequently become cuneiform. Of these intermediate plates the smallest is always that which is formed first; those which are formed later are always successively larger, whence it follows that the whole group of intermediate primary plates takes the form of a triangle, the apex of which, in the middle of the composite plate, only consists of the projecting extremity of the latest of them. It clearly results from all this, that these intermediate plates are in no way of a more recent origin than the others, that thev are neither secondary nor intercalated, but that they are successively formed, after the first entire plate, and before the last;" and Johannes Midler taught just the same. The formation of the secondary arcs is no less clearly explained, and is shown to be primarily due to the compression 'from above 1 ' Etudes sur les Echinoi'dees,' especially pp 21 et seg 2 A s iu Otimu. ' 3 T h e itaUos are |