are modifications of immigrants from Central America, marked x. They constitute 9.2 per cent, of the total. Five of the genera are modifications of immigrants from the ocean, marked O. They constitute 4.6 per cent of the total. There is no genus in the entire area whose derivation is in doubt. The fauna is largely a part of the general South American fauna which has been pinched off by the formation of the Andes and has gone its own way since the Andes have become high enough to form an effective barrier against the ready intermigration between the cisandean and transandean parts of the continent. The relation of the faunas of the different rivers in the area to each other is receiving consideration in separate papers.3 TABLE OF THE DISTRIBUTION OF GENERA. In the column "Origin" A Genera which are evidently modifications of present-day Orinoco or Amazon genera. C = Genera of the Pacific slope some of which are also in the Chagres, others in the Atrato. X = Genera of Northern, Central American origin. O Genera of brackish water origin. * = Pacific slope genera found only in the Atrato or Chagres of the Atlantic slope drainage. The dash (-) indicates that the genus occurs in the particular river. The addition mark (+) indicates that within the area the genus is limited to the one river. Columns 4 to 7 in sequence give a line of migration, II, 12, 13, 14, 15 give a different line, 11 being a duplicate of 4; columns 8, 9, 10 represent a fauna distinct from that in the Magdalena-Atrato-San Juan system shown in columns 3 to 7. The second column indicates the Maracaibo, Orinoco, or Amazon basins. 3 I regret to say, that so far, I have not been able to give consideration to the Santa river nor to the lower courses of the Rio Loa and to those of southern Peru. I hope that I may be able to visit these rivers in the near future. ANGUILLIDEÆ 83. Anguilla . PCILIDÆ 84. Gambusia. ATHERINIDÆ 91. Thyrina. 92. Menidia. SCIÆNIDÆ 93. Plagioscion. CICHLIDA 94. Geophagus 95. Aequidens. 96. Neetroplus . 97. Cichlasoma. GOBIIDÆ 98. Hemieleotris 99. Leptophylipnus. 100. Microeleotris. 101. Sicydium. 102. Pim lodus* 103. Hoplosternum. 104. Leporinodus 105. Abramites 106. Leporinus 107. Salminus. 108. Pæciliopsis Totals. Number of genera not in the *Numbers 102 to 108 are out of their regular places. 102 should go after 9, 103 should go after 21, 104-106 should go after 40, 107 should go after 72, and 108 should go after 90. This includes many genera widely distributed in the region east of the Cordillera of Bogota which west of them are found only in the Magdalena. GROWTH IN TREES.1 By D. T. MACDOUGAL. (Read April 21, 1921.) My studies of the course and physical conditions of growth were extended to include the changes in circumference and diameter of tree trunks as well as the elongation of growing branches in 1918, and a new technique with specially designed instruments was found necessary for the analytical study of the changes in volume of these massive organs. The records are now continuous for a large number of trees for many months, one tree having been under continuous measurement since September, 1919.2 Two new instruments were designed for obtaining measurements of growing trees. The dendrograph, an instrument for making continuous records of the variation of tree trunks, is an instrument consisting of a floating frame of metal of low temperature coefficient, such as invar or bario, which may be placed around a tree trunk, and the variation in distance between a contact rod on one side of the trunk and of one end of a rod or lever on the other side is traced by a pen on the free end of a lever on a sheet of paper carried by a recording cylinder. Such measurements are in terms of the diameter. In an earlier form of the instrument two levers were employed. One end of a bearing lever was placed in contact with the tree, the free end being linked to the short arm of the recording lever. The results previous to October, 1920, were obtained by 1 The full paper of which this is a synopsis will be published by the Carnegie Institution of Washington as prospective No. of its series. 2 MacDougal, D. T., "The Dendrograph; a New Instrument for Recording Growth and Other Variations in the Dimensions of Trees," Carnegie Inst. Wash. Year Book for 1918, pp. 59-60. "The Dendograph," ibid., for 1919, pp. 72-78. "The Coure of Growth in Trees as Measured by the Dendrograph," ibid., for 1920, pp. 51-52. "Measurement of a Season's Growth of Trees by the Newly Designed Dendrometer," ibid., for 1920, p. 52. |