A NUMBER of years ago I published in NATURE (December 25, 1890, p. 176) my opinion “On the Affinities of Hesperornis,” agreeing, at the time, with Prof. D'Arcy Thompson and others, that those toothed birds of the Kansas Cretaceous beds saw their nearest allies in existing birds in the Loons and Grebes, or in the typical Colymbidine assemblage. In other words, the now-living pygopodous birds, such as Urinator, Colymbus, and so forth, are, by descent, the modern representatives of the ancient Hesperonithidae, whether that descent or origin be direct or indirect. There are osteological characters, which the limitations of space will forbid dwelling upon here, that tend to convince me of the probability of the Grebes (Podicipoidea) being an earlier offshoot of the pygopodine stem than the Urinatoroidea, and so more nearly related to Hesperornis than the latter birds.