I. General Considerations. The Micrasters dealt with in this paper were collected from the sections exposed at Gravesend, the Thanet Coast, Dover, Beachy Head, the Dorset Coast, and Beer Head, in the zones indicated above. Two thousand examples have been measured and analysed, and no specimen has been included in the summary unless its zonal origin is accurately determined. Six hundred photomicrographio negatives of the special features of the test have been made, in order that mere conjecture may play no part in the enquiry, and that these important aids to specific and zonal determination may be placed on a permanent and scientific footing. The field-work has been extended over as wide an area as possible, so that conclusions should not be vitiated by being drawn from any one isolated district. In this paper, the term ‘low-zonal’ will apply to the zones of Rhynchonella Cuvieri, Terebratulina gracilis, Holaster planus, Micraster cor-testudinarium, and the lower third of the zone of M. cor-anguinum (80 feet); and the term ‘high-zonal’ will refer to the remainder of the zone of M. cor-anguinum, and to the zones above it Though all the accessible literature on the subject has been studied, the conclusions here set forth have not been influenced by previous publications, for rigid zonal collecting has been the paramount object, and the facts brought out by analysis have been allowed to speak for themselves. The scope of the present paper will not admit of the discussion of questions of nomenclature, which must be left