On Mean Relative and Absolute Parallaxes. By KEIVIN BURNS 496 The Nature and Origin of the Fishes of the Pacific Slope of Members who have not as yet sent their photographs to the Society will confer a favor by so doing; cabinet size preferred. It is requested that all correspondence be addressed TO THE SECRETARIES OF THE AMERICAN PHILOSOPHICAL SOCIETY 104 SOUTH FIFTH STREET PHILADELPHIA, U. S. A. OF THE AMERICAN PHILOSOPHICAL SOCIETY HELD AT PHILADELPHIA FOR PROMOTING USEFUL KNOWLEDGE THE ORIGIN AND DISTRIBUTION OF THE GENERA OF THE FISHES OF SOUTH AMERICA WEST OF THE MARACAIBO, ORINOCO, AMAZON, AND TITICACA BASINS.1 BY CARL H. EIGENMANN. (Read March 4, 1921.) The territory defined in the title includes Panama, Colombia, west of the Cordillera de Bogota, and the Pacific slope of Ecuador and Peru. The area is bounded on the south by the desert of Atacama, on the west by the Pacific Ocean, on the north by the Isthmus of Panama and the Caribbean Sea, on the east by high mountains, the Sierra Nevada de Santa Marta, the Cordillera de Perija, and the Cordillera of Bogota in Colombia, in small part by the Cordillera Oriental in Ecuador, and by the Cordillera Occidental in the rest of Ecuador and the whole of Peru. The largest river basin in this area is that of the Magdalena. The Magdalena, Sinu, Atrato, and Chagres drain into the Atlantic; the Chepo, Tuyra, San Juan, Dagua, Patia, Mira, Esmeraldas, Guayas, and many short turbulent rivers south of it drain into the Pacific. The coastal portion north of central Ecuador is very wet, with a heavy annual rainfall; the lower portion south of Guayaquil is without rain. The rivers are supplied with water from the mountains only. In a faunal volume recently finished, 388 species of fresh-water fishes are recognized from this area. They are referred to 108 1 Contribution from the Zoological Laboratory of Indiana University, No. 180. PROC, AMER. PHIL. SOC., Vol. lx, a, JULY 25, 1921 strictly fresh-water genera. What is the origin of this fauna? The solution to this problem is given by the distribution and relationship of the 108 genera. We find: 1. Sixty-five of the genera are also found east of the Andes where they are for the most part widely distributed. They are marked with the in the second column of the table below. The ancestors of the species of these 65 genera (66 if we include Rivulus, which is in part marine) had a common origin with the species found in the Atlantic slope rivers. They constitute about 60 per cent. of the total. 2. Twenty-eight of the genera are modifications of some of the above 65 or of other genera widely distributed east of the Andes. The derivation is in many cases quite evident and direct. For in stance: In a Xiliphius is a modified Bunocephalus; Cetopsorhamdia and Nannorhamdia are modified Rhamdia; Eremophilus is a modified Pygidium; Cheiridodus is a modified Plecostomus; Lebiasina is a modified Piabucina and a per cent. of individuals of Lebiasina still revert to Piabucina; Compsura and Pseudocheirodon are modified Cheirodon; Orthonophanes is a modified Brycon; Argopleura, Microgenes, and Phenacobrycon are modifications of Bryconamericus; Landonia is a modified Astyanax; Acestrorhynchus is a modified Acestrocephalus; Ctenolucinus is a modified Xiphostoma; and so on. number of other cases the immediate origin is not so evident: Parastremma and Rhoadsia form a distinct subfamily. Their young are in all technical respects members of the Cheirodontinae, from which they no doubt evolved. Gilbertolus is allied to the Characine; Genycharax to Astyanar or Charax; Grundulus and Phanagoniates are of the Cheirodontine, Pterobrycon and Microbrycon of the Glandulicaudine. All are marked A in the first column. They constitute nearly 26 per cent. of the total. Ten of the genera have either come from Central America or 2 Exclusive of the marine or brackish water genera, Pristis, Hexanematichthys, Sardinella, Stolephorus, Anchovia, Tylosurus, Mugil, Querimana, Agonostomus, Joturus, Centropomus, Pomadasys, Tarpon, Dormitator, Eleotris, Philypnus, Guavina, Gobius, Gobionellus, Awaous, Gobioides, Thalassophryne, Batrachoides, Citharichthys, Achirus. |