REVIEWS. 



Equid^ of the Oligocene, Miocene, and Pliocene of North 

 America : Iconographic Type Revision. By H. F. Osborn. 

 Memoirs of the American Museum of Natural History, New 

 Series, Vol. 11, Pt. I, 1918. pp. 217, with 54 plates and 173 

 text-figures. 



TT1HE work issued under the foregoing title is a systematic revision 

 -*- of the hundred and forty-six described species of Oligocene- 

 Pliocene horses of North America, with reproductions of the 

 original figures, new figures of many types, and a series of plates 

 of comparative drawings. It is intended to clear the ground for 

 President Osborn's long-looked for monograph of the Equidee 

 and is intentionally restricted to statements of fact with only that 

 irreducible minimum of theoretical conclusions essential in such 

 a work. 



In the opinion of the reviewer it is the most impressive palaeonto- 

 logical work which has ever been published. The short, absolutely 

 clear, and workable generic and sj)ecific diagnoses, the simple and 

 straightforward arrangement, the extreme lucidity of the com- 

 parative drawings, the abundance, perfection, and accurate dating 

 of the material, inevitably leave the reader with an overwhelming ■ 

 feeling that here the mode of mammalian evolution is shown in 

 detail for the first time, and shown in the best possible way by 

 arranging the facts in order and leaving them to speak for 

 themselves. 



When the final monograph is published, with President Osborn's 

 critical interpretations of the different phyla within each genus, 

 and of the evolution of each part considered in detail by itself 

 and in correlation with the rest of the structure, we shall have the 

 most adequate material which can at present be made available for 

 a critical discussion of biological theories of evolution. 



Palaeontologists and zoologists have been long acquainted with 

 the series of generic stages leading up to Eqiius, as they have been 

 established by the American palaeontologists ; but none except those 

 who have been so fortunate as to see the American Museum collec- 

 tions can have had any conception of the completeness of the story, 

 and even those who have handled that vast series could not 

 previously appreciate the infinitely close gradation between the 

 species of a single genus which occur at different horizons. 



As the importance of j^alaeontological material depends entirely 

 on the fact that the relative ages of the species forming a series 

 are known, President Osborn begins his monograph with a most 

 useful account of the stratigraphy and correlation of the many 

 localities of North America which yield horse remains. The whole 

 series of North American Tertiary land deposits is now divided 

 into zones on mammal evidence, and some progress has been made 



