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this process is uniformly and weaklv convex, forming a uniform continuation of the ventral
margin of the shell; its dorsal margin is, on the other hand, rather strongly concave. The rostrum
has a rather prominent, broadly rounded corner anteriorly; this corner is, however, less striking
because the ventral portion of the anterior margin of the rostrum, seen from the side, is covered
by a low, broadly rounded verruciform process, issuing ventrallv on the rostra of both valves
and directed forwards and downwards; the ventral corner of the rostrum is almost rectangular.
Seen from be low (fig. 4) the shell has its greatest breadth at about the middle; the
posterior siphon-shaped process is well defined. Seen from the front the shell appears
almost circular. The surface of the shell is without ridges and other sculptural
elevations except for the above-mentioned low, broadly rounded, verruciform process situated
ventrally on the rostrum and a low rounded elevation situated just ventrallv of the rostral
incisur, a short distance inside the margin of the shell. On the type specimen this last-mentioned
process was somewhat larger on the right valve than on the left one and covered the margin
of the shell to some extent when the shell was seen from the side; to judge from the larva (male)
that was also investigated, this character seems, however, to vary somewhat. The whole surface
of the shell is covered by numerous rounded cavities, which are situated rather close together,
are moderately deep and vary to some extent in size, those situated near the margin of the
shell being rather small and most of those situated farther in somewhat larger. In addition it
is to be noted that on the rostrum, on the part just behind the rostral incisur and on the siphon
the surface of the shell, near the margin, has diminutive warts. The pores of the surface are
small and moderate in number, either with or without a short, fine bristle. Seen from
inside (fig. 5): Medial bristles: On the rostrum there is a sparse row of rather long bristles
which runs obliquely forwards and upwards; most of these bristles are bifurcated and are
furnished with long, fine hairs arranged close together in the shape of a feather; the others
are generally simple and smooth. In addition on the rostrum behind and above this
row there is a moderate number of scattered, smooth, short, simple bristles. On the anterior
side of the incisur there are a few rather long or rather short bristles, most of them apparenti}’
simple and smooth. The two bristles situated close to each other near the inner edge of tbe
incisur are bifurcated, smooth or almost smooth, the anterior one being rather long; above these
there is a single, bifurcated, rather short, bare bristle. On the list behind the rostral incisur
there is a rather dense row of bristles, all rather long, bifurcated and furnished with long
hairs situated close together. and arranged in the shape of feathers (one of the anterior of
these bristles is reproduced in fig. 6). This row of bristles stops suddenly at about a third of the
distance along the shell; a few single, simple, smooth, rather short bristles may, however, be observed
on the list along the posterior part of the ventral side of the shell. A few, simple, short, smooth,
scattered bristles are to be found between the list and the inner line just behind the rostral incisur;
on the part between the list and the ventral margin of the shell no bristles seem to occur.
The inner walls of the posterior process of the shell, the siphon (fig. LI I), are almost perfectly
smooth, with only weaklv developed, reticulate sculpture (giving an apparent effect of scaliness)
and have a few scattered short simple bristles. Note also in fig. 1 the complicated structure
of the joining line and the marginal pores. The exit outwards of the siphon is a rather wide,
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