Two species of Chondrosteus have been described by Sir Philip Egerton, viz. Chondrosteus acipenseroides and C. crassior. Both were obtained from the Lias of Lyme Regis. The former species was known to Louis Agassiz, who named, but did not describe it (‘Poissons Fossiles,’ vol. ii. part 2, p. 280). In 1858 Egerton communicated a paper to the Royal Society (Phil. Trans. vol. xlviii. p. 871) on Chondrosteus, in which he described a number of specimens, which he referred to the species named by Agassiz and to the second one named above. The specimen which afforded the most perfect description was in a slab of indurated shale. A part of the head and the tail were missing; the length of the parts preserved was 15 inches, and the total length of the fish was estimated at 24 inches. This, together with examples of the head of C. acipenseroides, and a head and a good example of the tail of C. crassior, afforded a large amount of detailed information, on which Sir Philip Egerton was enabled to base the description and the determination of the genus. The characters of the species are indicated as follows (loc. cit. p. 883):—“The numerous specimens derived from smaller individuals present indications of two species, one having the bones of the head thin and smooth, the other being characterized by stronger cranial plates, having a granulated exterior. The anal fins of the latter are more massive, and have the transverse articulations at shorter intervals. I propose to