The examination of the internal structures of Jurassic fossils is attended with considerable technical difficulties. Even in cases where the surrounding matrix is sufficiently friable for the development of the external surface, the infilling material is usually either thoroughly indurated or irregularly nodular in texture. Internal moulds may be readily procured, both by natural weathering and artificial processes. But such preparations, though adequate for the investigation of shallow impressions such as the muscle-scars of Pelecypoda, are of uncertain service for the study of structures with amount of peristomial circumference (reduced to a simple circle) built up by the two areas is approximately the same, but the interambulacral portion is entrenched upon by the branchial incisions. The ambulacra present a semi-elliptical margin to the peristome, while that of the interambulacra is more nearly semi-circular. The ambulacral pores, which are not crowded nor much displaced from their direct line, pass down near to the adradial sides of the branchial incisions, and curve round with the margin of their areas.