To the student of Cretaceous palæontology there could be suggested, probably, no more puzzling question than that of the determination of the species of the genus Micraster. It seems not unlikely that Professor Forbes was of this opinion when, in 1850, he reduced to varieties of a single species—that of Micraster cor-anguinum—no fewer than seventeen previously supposed species, or subspecies, of the genus. It is true that, out of compunction, perhaps, for such wholesale slaughter, he forthwith established one new species, the Mieraster cor-bovis, and pointed out the probable existence of another —“a small Micraster with a very elevated extremity from the Chalk of Lyme.” This, however, seems scarcely to make up for the absorption or obliteration of all but one of the previously described species. And, seeing that several of these so-called varieties of Micraster cor-anguinum are recognized as species by many continental palæontologists, it becomes an interesting subject for inquiry whether our English Chalk does not contain more than one species in addition to the recognized Micraster cor-anguinum, and cor-bovis?