1906.] SELACHIAN FISHES. 727 



Thus in their essential features tlie Ohasmatopnea 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 Pleiu^opterygii, Acanthodii, and Ichthyotomi are exclusively 

 Palseozoic, whilst the Euselachii include all the living Sharks and 

 Rays. 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 wei-e 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 foi^mation 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. Fi'om 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 doi'sally 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 attach- 

 ment, thus permitting more varied movements in difierent 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 basijaterygium, 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 of 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 ; but 

 his memoir, although more elaborate than that of Thacher, is less complete. The I'e- 

 searches of Dohrn, Mayer, Dean, and others have developed and extended this theory. 



