1892. 



THE EVOLUTION OF FINS. 35 



ferous and Permian genera Xenacanthus (Fig. 7 and Fig. 8, i^) and 

 Pleuvacanthus (Fig. 8,/) also demonstrate how, by abbreviation and by 

 the atrophy of the rays on the hinder (or postaxial) border, the same 

 change has taken place ; and clearer proof of the identity of the 

 Selachian metapterygium with the axis of the so-called " archiptery- 

 gium " can scarcely be required. 



In short, so far as the facts of Palaeontology are known, the 

 evolution of the paired fins is essentially similar to that of the median 

 fins ; the only difference being that, while there are two different 

 modes of reduction in the latter, the crowding of the fin-supports at 

 two definite points is the sole process in the former. During this 

 process, until the " shortening up" of the fin-supports begins, there 

 is a tendency towards the contraction of the base of the fin to its 

 narrowest limits ; as soon as the shortening has made considerable 

 progress, the basal portion again becomes relatively broad — in the 

 pectoral fin, tribasal or polybasal. Just as the dermal rays in the 

 median fins arising thus are never correlated numerically with their 

 supports, so also the rays of the paired fins never bear any numerical 

 relation to their basal cartilages. The results being the same, the 

 causes are presumably the same ; and it is an interesting problem to 

 determine why both the dorsal fold and the lateral fold, when sub- 

 divided by concentration, invariably persists only at two points. 



REFERENCES. 



1. Gegenbaur, C. — Untersuchungen zur vergleichenden Anatomie der Wirbel- 



thiere, pt. ii., 18G5. 



2. GegenbauF, C. — Ueber das Skelet der Gliedmaassen der Wirbelthiere, Jena. 



Zeitschr., vol. v., 1870. 



3. GegenbauF, C. — Ueber das Archipterygium, ibid, vol. vii., 1873. 



4. Huxley, T. H. — On Ceratodus forsteri, with some Observations on the Classi- 



fication of Fishes, Pvoc. Zool. Soc, 1876. 



5. Balfour, F. M. — A Monograph on the Development of Elasmobranch 



Fishes, 1878. 



). Thacher, J. K. — Median and Paired Fins ; a Contribution to the Flistory of 

 the Vertebrate Limbs, Trans. Connecticut Acad., vol. iii., 1877. 



7. Thacher, J. K. — Ventral Fins of Ganoids, ibid., vol. iv., 1877. 



8. Mivart, St. G. — On the Fins of Elasmobranchii, Tyaiis. Zool. Soc, vol. x., 1878. 



9. Fritsch, A. — Fauna der Gaskohle, etc., des Permformation Bohmens, vol. iii., 



pt. i., 1890. 



10. Cope, E. D. — The Homologies of the Fins of Fishes, Antcr. Naturalist, 1890. 



11. Woodward, A. S. — Catalogue of Fossil Fishes in the British :\Iuseum, pt. iiy 



1891. 



Numerous references to other more special works and memoirs will be found 

 in those above cited. 



A. Smith Woodward. 



D 2 



