PREFATORY NOTE: The Ninth Volume of the Memoirs of the Carnegie Museum, herewith submitted, consists of four Memoirs. The first of these in the order of publication is a paper upon The Fresh-Water Fishes of Northwestern South America by Dr. C. H. Eigenmann, and was issued in October, 1922. The second is an article by Dr. Carl J. Drake, containing descriptions of the types of thirty-two new species and varieties of Neotropical Tingitidce. This paper was also issued in October, 1922. The third article is a paper upon The Skull of Diplodocus by W. J. Holland, issued as a separate in November, 1924. The fourth Memoir is a paper by Mr. O. A. Peterson upon the genus Dolichorhinus. This paper likewise was issued as a separate in November, 1924. It will be observed that a period of two years elapsed between the publication of Memoir No. 2 and Memoirs Nos. 3 and 4. This fact must be attributed to changes in the method of publication, which were made by the Board of Trustees of the Institute, who decided to install their own printingpress. This decision was accompanied by many and unavoidable delays, such as are incident to the initiation of all new enterprises. The first two numbers appeared with the imprint of the New Era Printing Company, Lancaster, Pennsylvania. The last two were printed by the Carnegie Institute Press. It is sincerely hoped that the various difficulties and delays attendant uj^on the installation of the new printing-press of the Institute will be overlooked by subscribers to the publications of the Carnegie Museum and we hope that from this time forward the publications of the Museum may be issued with the same promptness which has for the most part characterized their appearance during the many years which have passed since their publication was commenced. W. J. Holland, Editor Carnegie Museum November 10, 1924
