EURAPHIA HEMBELI. _Conrad._ Journal Acad. Nat. Sc. Philadelphia, vol.
7, 1831, Pl. 20, fig. 6.
_Shell dull reddish purple: sutures, when not obliterated, formed
by interlocking teeth: basis sometimes rendered calcareous by the
inflexion of the parietes: scutum with two or three furrows extending
down the middle of the valve._
_Hab._--California, near S. Diego, according to Conrad. Mus. Brit.,
Cuming.
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.