The book "Records of the Fossil Mammal Sinclairella, Family Apatemyidae, from the Chadron and Orell Ages, by William Clemens" is an important work in paleontology devoted to the study of fossil mammals of the family Apatemyidae from the Chadron and Orell Ages. The author, the eminent paleontologist William Alvin Clemens, presents the results of his research with a detailed description of the fossil discoveries of Sinclairella. The book contains a detailed analysis of the skeleton, fossils and morphology of Sinclairella, which expands our understanding of the evolution and biology of this extinct species. Clemens' work reflects a high scientific level of research in the field of paleontology and an important contribution to the development of the study of fossil mammals.