The fish fauna of the Cretaceous period is of most interest as exhibiting the last survivors of many of the ancient Mesozoic types and the dawn of the great host of modern forms which characterise the Tertiary epoch. Among sharks the Hybodonts and Cestracionts are becoming extinct, Cestracion itself alone passing into the Tertiary beyond, while numerous representatives of the characteristically modern group of Lamnidae suddenly make their appearance in the Gault, Greensand, and Chalk. Among other fishes the same kind of change is apparent, and it remains for future discoveries of intermediate forms to indicate the precise lines of evolution of which only widely separated terms are as yet well known. The surviving Jurassic families and genera that become almost or quite extinct in the course of the Cretaceous period are thus worthy of special attention. It is instructive to compare them with their early representatives at a time when such forms were a dominant type of life. The materials are often unfortunately fragmentary, but many noteworthy facts are discoverable in the attempt at comparison, and it is the object of the following notes to deal with some of the Cretaceous Hybodont and Cestraciont fossils from this point of view. Genus Hybodus. [Agassiz, Poiss. Foss., vol. iii., 1837, p. 41; A. S. Woodward, Cat. Foss. Fishes, Brit. Mus., pt. i., 1889, p. 250.] As represented by its typical species in the Lias, the principal characters of the skeleton of the genus Hybodus are now well-known. The paired fins and ...