I have seen five, old, large specimens, from an unknown locality, with their whole surfaces deeply corroded, and with most of the sutures obliterated; and three separated valves of a young specimen. From these materials, imperfect as they are, I feel no hesitation in identifying this species with the Euraphia Hembeli of Conrad, which is remarkable in several respects, and especially from being gigantic in size, compared to other members of its sub-family.
General Appearance and Structure of Shell.—The young specimen (fig. [5 b], about .7 of an inch in basal diameter) consists only of the carina, and the two lateral compartments; but these, as far as I can judge, resemble the specimen figured and described by Conrad, which was two inches in diameter. Shell depressed, spreading, surface moderately smooth, covered by brownish membrane: shell itself pale dull reddish-purple. The radii are not very narrow, with their summits rounded and very oblique: their edges are toothed, and their external surfaces are transversely ribbed, in correspondence with the interlocking points of the sutures. On the internal surface, the toothed suture is not visible, except near the base of the shell, owing to the overlapping of the alæ. The alæ have oblique summits, which are slightly notched owing to the upturned prominent lines of growth. The parietes are thick; their basal internal surfaces are rugged, with slightly branching ridges.
The old specimens (fig. [5 a]) are so much corroded that not a particle of the external surface is left: one of them which was 2-3/4 of an inch in basal diameter. Shell much depressed, spreading; colour pale purple; orifice large, rhomboidal, with a slight hollow on each side for the corners of the scuta: sutures generally in part or wholly obliterated; where still preserved, the interlocking toothed structure is distinct; the sheath is strongly marked by lines of growth, and is of a dark brown colour. The carina and rostrum are of unusually large size compared with the lateral compartments; and this, as far as I can judge, must have been the case with the younger specimens. That portion of the rostro-lateral compartment which forms part of the sheath, is reduced to a mere ridge. The most remarkable character is, that all these old specimens (of which the smallest measured nearly one inch and a half in diameter) had a flat, wide, calcareous basis, which is absolutely continuous with the inner lamina of the parietes, whereas in the younger specimen there was no appearance of any tendency in the parietes thus to grow inflected. There can be hardly any doubt that in a series of specimens some would be found with the parietes first forming a flat narrow ledge round the true basal membrane (as in the following species); and that in others, this ledge would be wider and wider, till its edges met in the middle, and coalesced into a continuous plate.
Opercular Valves.—I have seen these only in the old corroded specimens (fig. [5 c], [5 d]): they are locked together by remarkably strong articular ridges and furrows.
The Scuta have externally two or three impressed lines or narrow furrows, proceeding from the apex to the middle of the basal margin; these can be seen only in one of my specimens, owing to the degree to which the valves have suffered disintegration; but they are mentioned by Conrad. The basal margin is rather short compared with the other two margins. The tergal margin is remarkable from the depth of the upper furrow above the articular ridge, and from the size of the prominence (appearing like the true apex of the valve) above this upper furrow. There is a hollow for the adductor muscle, and traces of crests for both the rostral and lateral depressores. The Terga are generally but little corroded, and hence the dark brown membrane with which they are covered is well preserved; the shelly matter itself is also brown: there is only a trace of this colour in the more corroded scuta: the external surface of the terga is very smooth. The spur is pretty well developed, and is half as wide as the whole valve. The lines of growth are upturned along the carinal margin. The articular ridges and furrows are much developed. The crests for the depressor muscles are extremely strong; they depend beneath the basal margin, and are remarkable (fig. [5 c]) from being furnished each with fine sub-crests.
Animal’s body, unknown.
7. [CHTHAMALUS] INTERTEXTUS. Pl. [19], fig. [1 a], [1 b].
Shell, when well preserved, violet-purple: sutures, when not obliterated, formed by oblique interfolding laminæ: basis membranous, but surrounded by a ledge formed by the inflected basal edges of the parietes: scutum and tergum completely calcified together.