The material for this work was that collected and prepared by Professor J. Graham Kerr in the Chaco (Lepidosiren), and by Budgett from the Gambia (Protopterus). The process of development follows nearly the same course in the two genera, and where differences occur they are mainly such as are to be expected from a comparison of the adult skulls. While the series of stages ofLepidosirenleft no unfilled gaps, the material was scantier in the case ofProtopterus. Except where otherwise stated, the

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... etailed descriptions were worked out inLepidosiren, but whenever I was unable to verify that the process was substantially the same in the other genus, the fact has been mentioned. The stages are numbered in correspondence with the stages figured by Graham Kerr (Phil. Trans. Roy. Soc, B., vol. cxcii.). TheProtopterusstages are numbered the same as the corresponding ones in the other genus. As far as possible, the nomenclature adopted by Gaupp* has been adhered to.