Shell oval in shape, very thin, consisting of six whorls, the last one very large and inflated. In some specimens the spire is rather prominent, in others slightly depressed with rather deep sutures. The apex and protoconch when not broken off are pointed and prominent. The aperture is broad in front but narrow behind, and seems to have a faint shallow anterior channel. There is no columellar fold. The outer lip in specimens where it is preserved is sharp and very slightly flattened out and expanded anteriorly. The growth-lines are fairly prominent, and there is a tendency for faint and shallow parallel furrows to develop, especially on the anterior portion. Height about 18 mm.

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.

Leidleite and Inninmorite.—Typical rocks of these groups are as yet unknown in the Clyde area. A N.N.W. dyke at Barrassie Sands, near Troon (Ayrshire), has points in common with leidleite. It shows anorthite phenocrysts in a subvariolitic groundmass of acicular felspar and augite, enveloped in a partially devitrified glassy base. A north–south dyke at the north end of Loch Fad, Bute, may perhaps be regarded as a doleritic end-variety of leidleite (see Anderson & Radley, 1916, fig. 2b, p. 208). A pitchstone-like dyke from the Smurig Burn in the south of Arran has all the characters of leidleite, except that it carries rather abundant granular augite, instead of acicular crystals.

In the following pages the writer has endeavoured to consider this skeleton in a common-sense way, and to arrange the limbs with reference to ordinary mechanical principles, and also by comparison of the bones with those of different mammalian and reptilian types. A good many years ago, Von Meyer was so struck by the colossal and rather straight hind-limb bones of the Dinosauria and their superficial resemblance to those of elephants, that he proposed the term Elephantopoda; but at that time the group had not been classified by Marsh and others into distinct sub-orders, with very different limbs.