4o8 SCIENCE PROGRESS 



rise to a fin-element, consisting of two rods of cartilage 

 (somactidia), with two dorsal and ventral bundles of muscles, 

 each supplied by a spinal nerve. These fin-elements disappear 

 except in the pectoral and pelvic regions, where a considerable 

 number of these outgrowths are concentrated, and unite to 

 form the pectoral and pelvic fins respectively. Now here 

 we find an instance of a great number of similar structures 

 (somactidia or segmental radials) in the fin-fold ; as a result 

 of this multiple repetition there naturally ensues a high degree 

 of variability, and the first step in the successful variations 

 consisted in the breaking up and reduction of the continuity 

 of the fin-folds into a series of paired fins, forming a double 

 series along the entire length of the abdomen. Some of these 

 earlier fins, lying between the pectoral and pelvic fins, be- 

 came reduced to mere spines, which Smith-Woodward ^ re- 

 gards as the stiffened front-edges of such fins, e.g. in the 

 Acanthodian Climatiits scutigcr, Egerton, and MesacantJms 

 MitcJiclli, Egerton, of the Lower Old Red Sandstone (figs. 2, 3). 



The next stage, following the differentiation of the con- 

 tinuous fin-folds into separate fins, was that in which the 

 fin-flaps attained greater rigidity by the concrescence of the 

 bases of the somactidia at four nodal points. Now the cera- 

 totrichia are supported by tri-segmented radial cartilages 

 (pterygiophores). This primary number three is of consider- 

 able significance, for the pentadactylate limb is essentially 

 composed of three segments, the foot being regarded as the 

 distal segment, which has become still further segmented 

 secondarily. In Cladoselache — the most primitive form with 

 regard to the appendicular limbs — this tri-segmented nature 

 of the fin is clearly visible, but even here some reduction has 

 evidently taken place in comparison with the simpler arrange- 

 ment still visible in the unpaired fins of living Elasmobranchs. 

 In the pectoral fin of Cladoselache^ (fig. 5), the elements of 

 the basal segment have become fewer and stouter, by fusion 

 due to growth-pressure, so as to form a basal support (basi- 

 pterygium), and the distal elements have become more slender 

 and reduced in number. But no trace of pelvic girdle was 

 developed in Cladoselache, and the pectoral girdle did not 



^ Presid. Address, P/vc. Geol. Assoc, xix. 1906, p. 266. 



2 B. Dean, Joiirn. Morph. ix. 1894, p. 87; Trans. Neiu York Acad. Sci. xiii, 

 1894, p. 115 ; and AfuU. A>iz, xi. 1896, p. 673. 



