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Show 58 PROF. T. H. HUXLEY ON CERATODUS FORSTERI, [Jan. 4, recollected that known forms certainly represent but a portion, and probably a small portion, of those which have existed, and that the most natural groups are therefore, to a great extent, the result of the influence of extraneous, and what may be properly termed accidental, conditions. It has occurred to me that, in the present state of science, it is very desirable to have some mode of stating the facts of morphology in a condensed and comprehensible form, which shall be purely objective and free from speculation ; and I now proceed to illustrate my meaning by drawing up a scheme of the morphology of the Ichthyopsida. Looking at the animals included under this head as a whole, or at the development of any of the higher members of the group, it is observable that they present a certain series of stages of differentiation marked by the broad characters of the skull, the nature of the olfactory and respiratory organs, and the development or non-development of an opercular fold of the integument. Thus the skull either retains its primitive segmentation (Entomo-crania), or the primitive segmentation is lost, and a chondrocranium is developed (Holocrania). There are two external nostrils (Am-phirhina) or only one (Monorhina). A pneumatoccele, or air sac, which may become either an air-bladder or a lung, is developed (Pneumatoccela), or not (Apneumato-ccela) ; and a fold of the integument may cover the branchial apertures (Operculata), or not (Inoperculata). The Ichthyopsida also exhibit a series of stages of differentiation of the limbs, being either apodal or pedate; and, when pedate, having the limb-skeleton constructed upon the type of the archipterygium, or on that of the icthyopterygium, or on that of the chiropterygium. Moreover, when the limb is an ichthyopterygium, it may possess one, or at most two basal elements, which articulate with the pectoral arch (unibasal), or there may be three (tribasal), or there may be many (multibasal), in accordance with the greater and greater divergence of the fin from the archipterygial type. The chondrocranium may be constructed upon either the amphistylic, the hyostylic, or the autostylic plan. Now, if the stages of general differentiation be indicated by points on a vertical line from which horizontal lines are drawn, and the stages of subordinate differentiation of the skull and limbs be indicated by points on a horizontal line from which vertical lines are drawn, we shall have vertical series of intersections indicating general differentiation, and horizontal series of intersections indicating special differentiation. Every known form will occupy some given intersections, and the unoccupied intersections will indicate unfulfilled, or unknown, possibilities of organization. The following Table exhibits the groups of the Ichthyopsida arranged according to this scheme. |