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Show 1 9 0 6 . ] SELACHIAN FISHES. 7 2 7 Thus in their essential features the Chasmatopnea are the more specialised, but on the other hand they are in some respects more primitive than any other living Selachians. Of the Trematopnea, the Pleuropterygii, Acanthodii, and Ichthyotomi are exclusively Paheozoic, whilst the Euselachii include all the living Sharks and Kays. These orders rest solely on what is known as to the structure of the paired fins, and our views as to their relationships are determined by our conception of the evolution of those organs, which must therefore be discussed. The view which is here taken as to the origin and evolution of the paired fins in the Selachians is as follows:- The median and paired fins were originally continuous and were supported by series of parallel cartilaginous rods-pterygio-phores- set at right angles to the axis of the body. At the line of junction of fin and body each rod became segmented ; thus we get a differentiation into basals-the proximal segments within the body, and radials-the distal segments. The radials often became subdivided, a series of short " marginal " segments being the most constant. Hypertrophy in certain regions and atrophy in others led to the establishment (at least in the Euselachii) of two dorsals, a caudal, an anal and paired pectoral and pelvic fins. In the pelvic fins concentration and fusion of the anterior basalia on each side led to the formation of a pelvis (except in the Pleuropterygii); in the Euselachii these united to form a single unpaired cartilage, and some or all of the remaining basalia fused to form a basipterygium. From their position of greater importance, evolution has proceeded further in the pectoral than in the pelvic fins; the cartilages formed by the fusion of the anterior basalia have grown out dorsally and ventrally to form the pectoral arch, and the normal course of evolution of the fin has been in the direction of shortening the base of attachment, thus permitting more varied movements in different planes. This shortening of the base has not been accomplished by a simple concentration and reduction of elements, as in the Teleostean Fishes, but by the outward rotation of the basipterygium, which has retained its anterior articulation to the pectoral arch, but posteriorly has separated from the body and has come to lie at the posterior (inner) edge rather than at the base of the fin. Evolution in this direction has proceeded furthest in the Ichthyotomi, in which the posterior radials have extended round on to the inner edge of the segmented basipterygium. It is now nearly thirty years since Thacher and Balfour* independently put forward the theory that the median and paired fins were or similar origin, both being the remnants of originally continuous fins. The former based his view on the similar structure of the median and paired fins in the Selachians and the Chondrostean Fishes, whilst the latter came to his conclusions * I am not overlooking the fact that Mivart also put forward this theory; hut his memoir, although more elaborate than that of Thacher, is less complete. The researches of Dohrn, Mayer, Dean, and others have developed and extended this theory. |