460 



AMERICAN MUSEUM OF NATURAL HISTORY 



Fins — Cont'd. 



Excellent plates of the skeleton of the 

 median fins of many Selachians appear in 

 •Mivart, St. G. 1879.1. 



Heterocercy in Teleostomi 



" In Acipenser the typical heteroceTcal form 

 ... is present; Polydon by an increase in the 

 size of the ventral lobe reaches a step towards 

 the externally symmetrical homocereal fin; 

 while Atnia proceeds furthest of all in_ the 

 homocereal direction, requiring but one single 

 feature, the possession of a true urostyle, to 

 attain the homocereal condition." Whitehouse, 

 R. H. 1910.2 (p. 595, 596). 



Development of median fins, especially 

 the dorsal fin, in Acipenser. Dikson, B. 

 1912.1; •Zalenskii,V.V. 1899.1; Zograf, 

 N. Y. 1887.3; Schmalhausen, J. J. Add. 

 1913.1. 



Development of tail, respectively in Lepi- 

 dosteus and Amia. Wilder, B. G. 1876.2, 

 1877.6. 



General accounts of the tail of Teleos- 

 tomes, including Ganoids, are Heckel, J. J. 

 1850.3, and Kolliker, R. A. Add. 1860.1. 



Homocercy. With the possible exception of 

 the forms mentioned below under " gephyro- 

 cercy," this form of caudal is found in all Teleos- 

 tei. It may be considered a specialized form of 

 the heterocercal type. The term " homocereal " 

 is very broad in application. By Whitehouse 

 (R. H. 1910 J, p. 138), it is defined as " a caudal 

 fin which is externally symmetrical but in which 

 the majority of the fin-rays are supported in- 

 ternally by hypaxial [ventral] elements, and in 

 which a urostyle is present either in the adult 

 or at some time during larval history." 



The most comprehensive account of the 

 morphology of the tail in Teleostei is *White- 

 house, R. H. 1910.2. 



_^ Huxley (T. H. 1859.1) says that 

 " the first accurate and comprehensive ac- 

 count of the structure of the piscine tail," is 

 •Heckel, J. J. 1850.3. 



Other papers on the morphology or de- 

 velopment of the caudal in Teleosts, are 

 •Agassiz, A. 1878.1; Baudelot, E. 1868.2; 

 Costa, A. 1869.1; •Huxley, T. H. 



1859.1; •Lotz, T. 1864.1; Peach, C. W. 

 1859.2; Regan, C.T. 1910.2,.5; •Ryder, 

 J. A. 1885.11, .14, 1886.8; Sauvage, H. 

 E. 1872.1; Schmidt, V. 1893.1; Steen- 

 strup, J. J. 1866.1; •Totton, A. K. 

 iPleuragramma) 1914.1; •KoUiker, R. 

 A. Add. 18G0.1; Newton, E. T. Add. 

 1882.1. 



Gephyrocercy. The gephyrocercal caudal is 

 a secondary one, the original caudal being 

 aborted or lost during larval (Mola) or post- 

 larval existence (Fierasfer), and the secondary 

 fin formed by the union of the rays of the dorsal 

 and ventral median fins, around the abbre- 

 viated extremity. It is perfectly symmetrical, 

 both externally and internally. The term 

 " secondary diphycercy," as above stated, is 

 synonymous with this term. 



As stated in the original definition (Ryder, 

 J. A. 1886.8, p. 991), among Teleosts, this type 

 of caudal occurs only in the two forms, Mola 

 and Fierasfer. 



Certain fossil forms of Ccelacanthidfe (Cros- 

 sopterygii), such as Undina, possess a gephy- 

 rocercal caudal fin (Reis, O. M. 1888.1, 1892.3). 



Gephyrocercy in Dipnoi 

 " It is highly probable that this group will be 

 found to afford the best examples of the gephy- 

 rocercal form." Whitehouse, R. H. 1910.1, 

 p. 142. 



" Diphycercy in its existing conditions, . . . 

 as in Ceratodus, is, ... a specialized, _ per- 

 haps more strictly a degenerate condition, 

 directly comparable with gephyrocercy." Dean, 

 B. 1894.1, p. 102. 



Dollo (L. 1896.2) in the " phylogSnie des 

 dipneustes " has erred in too extensive an ap- 

 plication of the term "gephyrocercy," in the 

 belief of Whitehouse. 



PAIRED FINS 



Including the pectoral, and the pelvic 

 or ventral fins. The following references are 

 arranged primarily with regard to the anatom- 

 ical structure in the different groups. However, 

 most of such researches have been inspired by 

 the views of the derivation or origin of the 

 paired fins. The latter topic forms a separate 

 subject and the chief hypotheses on the origin 

 of the Umbs are discussed below " Phylogenesis 

 of limbs " under Evolution. 



The hypothesis of the origin of the paired 

 fins from a continuous lateral fin-foid, like that 

 of the median fins, is now the most prevalent 

 view. It was the independent observation by 

 both Mivart and Thacher, of the resemblance 

 between the median and paired fin-structures 

 in Raja, which led each separatelyto form his 

 conception of the identity in origin of the 

 median and paired fins (Mivart, St. G. 1879.1, 

 p. 467). 



General treatises 



Valuable general treatises in German, 

 treating of the development and structure of 

 the paired fins in all groups, are •Braus, 

 H. 1904.2, 1906.1; •Davidoff, M. 1879.1; . 

 •Gegenbaur, C. 1864.2 (ii); and •Wie- 

 dersheim, R. 1892.1. 



Valuable treatises in English, especially 

 reviewing the origin of the paired fins, are 

 •Goodrich, E. S. 1906.1 and •Mivart, 

 St. G. 1879.1. 



Various miscellaneous or unclassified 

 papers on the paired fins are Geoffroy- 

 Saint-Hilaire, E. F. 1807.3,.4,.7, 1818.2,.4; 

 Gouriet, E. 1868.1; Hartwig, P. 1870.1; 

 Hatsohek, B. 1889.1; •Kner, R. 1860.2; 

 Lavocat, A. 1896.1; Mettenheimer, C. F. 

 1847.1. 



Paired fins in the 

 various groups 



Fins of Dipnoi 



Gegenbaur (C. 1870.4) originally adopted 

 the Lepidosiren type of limb as the ancestral 

 form of the Elasmobranch fin. He later (1873.1) 

 adopted the Ceratodus type as the archiptery- 

 gium or the fundamental form of the verte- 

 brate limb. This view was accepted by Giin- 

 ther (A. C. 1871.4) and by Huxley (T. H. 1876.1) . 



A contrary view was formulated by Mivart 

 (St. G. 1879.1, p. 473) who says " the fin of 

 Ceratodus cannot represent the archipteryg- 

 ium, and, far from being a primitive form . . ., 

 is, on the contrary, a very special and peculiar 

 structure, which is carried to a still more ab- 

 normal development in Lepidosiren, by pro- 

 gressive elongation and by atrophy of the post- 

 axial radials." 



Howes (G. B. 1887.1, p. 24) believes "that 

 the paired fins of Plagiostomes and Dipnoi 

 have, in all probability, arisen independently 



