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Show • ] MR. F, J. BELL ON A NEW GENUS OF ECHINOIDS. 45 aphorism " The development of the individual is a compressed epitome of the development of the race ;" and we may further look for an explanation of the suddenness of the changes in the supposition that between definite points in organization neither the larval form nor the adult are enabled to maintain that equilibrium in the presence of external forces which is necessary to the maintenance of existence1. Whether this be so or not, neither "the doctrine of Descent nor the "fundamental principle of biogenetic development" has its truth in any way affected thereby. It is with considerations of this kind in our mind that we must, as I think, address ourselves to the consideration of intermediate forms ; for, in our times at least, it is only when observation is kindled by the light of the doctrine of the Descent that the full value of "inosculant forms" can be justly estimated. Description of the Specimen. Ambulacral system.-The paired arrangement of the ambulacral pores does not extend beyond the ambitus, which is very nearly reached by all the five sets, but most completely by the two posterolateral ; the two rows of each are, in the case of the postero-lateral and of the anterior ambulacrum, altogether equal; but in the anterolateral ambulacra the anterior row of pairs of pores is a little shorter than the posterior, and this difference is best marked on the left side ; the strictest parallelism is observed between the paired rows, which incline so slightly towards one another that the diminution in breadth of the intraambulacral space cannot be detected by the eye until the pores come close to the central or apical system; here the pores diminish considerably in size, but there is no bare space separating the perforated ambulacral plates from the azygos radial plate, ordinarily known as the ocular. The pores of the inner row in each pair are still fairly circular; those of the outer are more slit-like or comma-shaped; and it is evident, so soon as several different pores have been examined, that the specimen in question exhibits a commencement of that union of the two pores by means of a connecting furrow which is very much more, and quite distinctly, marked in Echinolampas, and is even to be seen in Conoclypeus leskii, Goldfuss2. The spacing-out of the ambulacral pores as they approach the point at which the paired arrangement ceases to obtain is here but barely marked; and, indeed, it would be impossible to detect it at all, were we not led to look for it from the marked degree which it reaches in Echinolampas; so, again, while it is in some cases possible to see that the terminal pores of the outer row of the ambulacra are as completely circular as those of the inner row, and so far to find an analogy with the much more marked similarity in Echinolampas, yet in the cases of other rows on the same i The probability of sports leading to very considerable and remarkable changes in organization has, comparatively lately, received support from the observations of M r . Bullar on the hermaphroditism of certain parasitic Isopoda (Journ. Anat. & Phys. vol. xi. pp. 118-124). a ' Petrefacta Germanias,' tab. xiii. figs. 1, a-c. |