Twitchell, n. sp.
figures 1a-f, 2.
characters.—Test variable in size; subcircular to subtriangular in
marginal outline, broader than long, broadest posteriorly; the whole
form greatly depressed, being thin and nearly flat around a narrow
marginal area within which it rises very gradually and gently to the
low, central apex; margin thin, with a small notch in the middle of
the posterior edge; under surface flat. Ambulacral petals elliptical,
extending about half way to the margin. Apical system central.
Peristome small, circular; ambulacral furrows simple and straight for
two-thirds or more of the way to.the margin, thon apparently forking
symmetrically. Periproct very small, circular, almost at the ambitus
though slightly inframarginal, at the edge of the notch in the middle
of the posterior margin.
A: Length 50 millimeters; width 52.5 millimeters; height 6.
millimeters. Specimen B: Length 52.5 millitheters; width 76
millimeters; height S millimeters.
new Scutella is one of the most abundant in the American Cenozoic
deposits. The writer bas had opportunity to examine several hundred
specimens. Neirly all of these were from the Eocene deposits of
Mississippi, hence the specific name. The test is variable in size,
ranging from less than inch to over 3 inches in diameter and from less
than 1/8 to nearly 1/2 inch in height. The marginal outline is also
quite variable, ranging from circular to subcircular in the younger
forms to subtriangular and even transvcrsely elliptical in the larger
forms. The whole form is much depressed, being thin and nearly flat
around a narrow marginal area, although slightly more so posteriorly
thon elsewhere, from wich it rises gradually and gently to the low,
central apex. The margin is thin and faintly undulating, with a small
notch in the middle of the posterior edge; the under surface is flat
cxcept around the peristome, where it is slightly concave. The apex is
central or subcentral, at the summit of the broad central tumid area.
The ambulacral areas are
narrow in the petaloid region but wide in the marginal region where
they arc wider thon the interambulacral areas. The dorsal portions of
the ambulacra are petaloid. The petals are subequal in length, the odd
petal slightly longer and narrower thon the rest; elliptical in form,
slightly open at the ends and extencl about half way to the margin.
The poriferous zones are rather wide, about equal to the.
interporiferous areas, though varying somewhat both among the petals
on a single specimen or on different specimens, being slightly wider,
equal to, or slightly narrower than the interporiferous areas; the
limer row of porcs oval, outer row slitlike, pairs of pores
conjugatcd. A few pairs of pores occur beyond the ends of the petals.
The interambulacral arecs
are large though somcwhat smiller both between the petals and et the
margin than the ambulacral arecs. The whole surface of the test is
covered with small uniform tubercles which are slightly larget. on the
The apical system is
subcentral, at the summit of the central tumid area. The madreporite
is relatively large, occupying the langer part of the arec covered by
the system, and flush with the surface. There are five genital pores
near the edge of the madreporite. One specimen studied possessed the
abnormal feature of a double posterior genital pore. There are five
small radial plates, each perforated by a minute pore.
The peristome is very
small, not over double the size. of the periproct, circuler, central
or slightly excentric posteriorly; the ambulacral furrows simple and
straight for two-thircls or more of the distance from peristome to
margin, thon apparently forking symmetrically, each pair of branches
continuing almost to the margin. The under side is revealed on few
specimens, and on few can the furrows be made out.
The periproct is very
small, circuler, almost at the ambitus though slightly inframarginal,
situated et the, under and inner edgc of the notch in the middle of
the posterior margin.
mississippiensis does not appear to be closely related to any
other American form. It resembles Periarchus lyelli, in general
appearance, and specimens have doubtless been erroneously labeled
S. lyelli, in some collections; S. mississippiensis,
however, is readily distinguished by its submarginal periproct,
greater width, the notch in its posterior edge, and the fact that its
tumid central area involves nearly the whole upper surface. It bears
an interesting resemblance to the recent species Scutella parma, but
differs from it in having less widely open petals, much narrower
interporiferous arecs, a less circuler and more anteriorly pointed
form, and in not having its periproct exactly marginal or et the
Of foreign forms S.
patagonensis (Desor) Ortman, cspecially the "alate" form described
by Ortman and fornierly known as Echinarachnius juliensis Desor
from the Neocene of Patagonia, appears to be most closely related to
S. mississippiensis. It is very siinilar in marginal outline,
location of periproct, etc., but differs in having e less central
apical system, longer petals, narrower interambulacral arecs, and in
having its ambulacral furrows bcghming simply and thon forking in
symmetrical curving branches almost immediately. S. subtetragona
De Grateloup and S. striatula M. de Serres, both from the upper
Eocene of France, aise present certain points of identity with S.
mississippiensis. Each, however, differs from the American form in
having four genital pores instead of five and in having the ambulacral
furrows foré near the peristome.
Branch (type), Radiate Banks, Spillman's Clam beel and other
localities in and around Enterprise, Clarke County, Miss.; San
Augustin, and 1 mile southwest of Palestine, Tex.
group, middle Eocene. In Mississippi, et the base of the Lisbon mari.
In Texas, hi the marine substage of the Claiborne.
National Museum. (137655, A; 137655a, B); Wagner Free Instituts of