Wright. Pl. XV, fig.4 a, b, c, d, e, f, g, h, l, m.
and Magazine of Natural History, 2d ser., vol. viii, p. 161, pl.
11, fig. 1.
of the Geological Survey, Decade IV, l. 2.
Morris's Catalogue of British Fossils, 2d edit., p. 70.
des Echinides Fossiles, p. 144.
hemispherical, more or less depressed, sometimes elevated ; ambulacral
areas narrrow, undulated, with two rows of small, perforated tubercles,
fourteen to sixteen in each row, which alternate on the borders of the
area, and gradually diminish in size from the base to the apex ;
inter-ambulacral areas with large prominent tubercles on the sides,
and small tubercles near the disc ; arealas sub-confluent ; miliary
zone narrow, with two rows of granules ; apical disc very large ;
sur-anal plate composed of six elements ; mouth large, decagonal ;
peristome deeply notched ; primary spines twice or more in lenght the
diameter of the test ; stem sub-angular, tapering, or slightly
compressed ; extremity sometimes bifid or trifid.
Dimensions. - This species varies so much in size and figure, that I
have selected four specimens on account of their differences, the
comparative dimensions of which are shown in the following table :
Common small form.
Transverse diameter of test
Height of test .
Diameter of mouth opening
Length of apical disc
Description. - This is the most common and best preserved of all
our fossil sea-urchins ; it has long been known as a Cornbash species,
but was neither named, figured, nor described, until I gave its
history, with figures and details, in my "Memoir on the Cidaridae of
the Oolites ;" since then it has been figured and described by
Professor Forbes in the fourth Decade of the "Memoirs of the
Geological Survey;" nd by M. Desor, in tabl. XX, figs. 19-23, of his
valuable "Synopsis des Echinides Fossiles." This species
exhibits much variation in size and figure, but its diagnostic
characters are preserved with remarkable uniformity throughout these
different phases of form and magnitude.
The test is sometimes elevated and globular, like a Hemicidaris
; indeed, the large forms, the dimensions of which are given in the
first column of the table of measurements, are commonly so named ; the
absence of semi-tubercles at the base of the ambulacra, and the large
size of the elongated discal opening, are the only characters by which
they ca be distinguished from that genus. In the more common form
(fig. 4 c), the body is spheroidal and depressed on the upper
surface ; and, when the flattening is excessive, it produces the
depressed form of the third column.
The ambulacral areas are narrow and moderately prominent (fig. 4 a)
; they are nearly of a uniform width, gradually expanding in the lower
half, and tapering in the upper ; they exhibit a very slight
undulating contour, and have two rows of small, nearly equal-sized
secondary tubercles, from sixteen to eighteen in each row (fig. 4 c)
; the tubercles at the basal angle are the largest (fig. 4 b,
e), and they imperceptibly diminish in size from the
circumference to the disc (fig. 4 a), they are all perforated
and raised on bosses, which have ten crenulations on their summits
(fig. 4 d) ; the tubercles of each row alternate, and in the
centre of the area there are one or two rows of granules, which send
off small lateral branches to encircle the areolas (fig. 4 d, e).
The inter-ambulacral areas are nearly four times as wide as the
ambulacral ; they consist of two columns, each composed of seven or
eight plates, each plate bearing a primary tubercle (fig.4d) ;
the two plates nearest the peristome are very small (fig. 4b),
the four on the sides are very large (fig.4c), and the two near
the disc are small(fig.4a) ; the tubercles are elevated on very
large bosses (fig.4f), which have ten or more deep crenulations
on their summits, the tubercles themselves are perforated ; around the
base of the boss there is a wide, smooth areola ; the plates are
bordered by a single row of granules (fig.4d), which, however,
is sometimes absent from the upper and lower borders, the areolas then
being confluent ; the miliary zone is narrow, and is formed of two
rows of granules, with the intermediate angles having a few smaller
additional granules introduced ; the areolas are separated from the
poriferous zones by a row of granules on the zonal side of the plates
The poriferous zones are narrow, the pores unigeminal throughout,
except at the base, where they fall into triple oblique rows ; the
septa form small elevations on the surface, and a beaded line thereby
passes down the zone between the pores forming a pair (fig.4d),
and there are nine or ten pairs of pores opposite each tubercular
The apical disc is often admirably preserved in this beautiful urchin
; the study of its curious structure first enabled me to correct M.
Agassiz's erroneous supposition, that the genital plate, which carries
the madreporiform body, is the single plate, and represents the
posterior side of the animal, instead of the right antero-lateral
plate which occupies the same relative position in all the
The disc is one third the width of the test, and in cosequence of the
projection of the genital plates has a pentagonal form ; it is
slightly convex and prominent, the anterior and posterior pair of
genital plates are nearly of the same size (fig.4 a, i), the
right anterior is the largest, and carriest on its front part the
madreporiform body ; the small crescentic-shaped single plate forms
the posterior boundary of the vent (fig. 4i) ; the oviductal
holes are all near the apices ; the sur-anal plate is composed of xis
or seven pieces arranged like mosaic before the anal opening ; the
ocular plates are small and firmly wedged between the genital and
sur-anal elements. All the plates of the apical disc have numerous
granules scattered on their surface.
The mouth opening is wide, nearly one half the diameter of the text ;
the peristome is decagonal ; the ambulacral being larger than the
inter-ambulacral lobes, the ten deep notches, with reflected edges,
indent the bases of the inter-ambulacra, and extend as far as the
areolas of the second or third tubercles.
The primary spines are finely preserved in situ in fig. 4 k ;
they are variable in size in the same and in different specimens, and
are proportionally shorter in young than in old urchins ; sometimes
they are three times the length of the diameter of the body (fig.4
b), and are sometimes nearly three inches and a half in length ;
the head is conical, with a truncated extremity marked by deep
creulations (fig.4 m), the ring is prominent, and the milling
is angular and sharp ; the stem (fig. 4b) swells out beyond the
ring, it is more or less irregularly sub-angular, with the angles
rounded ; a transverse section of one of the spines exhibits an
irregularly elliptical figure ; sometimes the spine tapers to a
conical point, or the distal end terminates in a bifid or trifid
extremity. The secondary spines articulating with the ambulacral
tubercles (fig. 4 n) are short, about three tenths of an inch
in length, they are round, and taper gently from the ring to the
point, their surface being covered with fine longitudinal lines (fig.
The jaws are preserved in one or two specimens (fig. 4 g) ; the
lantern is strong (fig.4 h), the teeth conical, and in its
general appearance the dental apparatus closely resembles that of an
Affinities and differences. - This urchin very much resembles a
Hemicidaris ; in fact, Acrosalenia and Hemicidaris
have so many characters in common, which are always well preserved,
and so few that are diagnostic, and which for the most part are either
broken or absent, that it requires considerable practice, when the
apical disc is wanting, to determine the genus ; the absence of semi-tubercles
at the base of the ambulacrea and the magnitude and pentagonal form of
the discal opening serve as good guides to the genus. Acrosalenia
hemicidaroides is distinguished from A. Lycettii by having a
larger and more spheroidal test, larges bosses on the sides, and
smaller ambulacral tubercles ; the apical disc is larger, and the
sur-anal plate is composed of a greater number of pieces.
Acrosalenia hemicidaroides is distinguished from A. pustulata
in having regular rows of secondary tubercles in the ambulacral areas,
which are larger and more fully developed ; the miliary zone is
narrower, with only two row of granules ; the primary tubercles are
more developed at the upper surface, and the apical disc is likewise
The same characters distinguish it from A. Wiltonii ; but
besides those already enumerated, that species has four rows of fine
granules in the miliary zone, smaller tubercles, suddenly diminishing
at the upper surface, and a much smaller mouth opening. The complex
character of the sur-anal plate, the size of the test and of its
primary tubercles, readily separate it from A. spinosa.
Locality and Stratigraphical position. - The finest specimens
of this urchin with their spines attached (fig. 4 k) have been
collected by Mr. William Buy, from the Forest Marble, near Malmesbury,
in a thin vein of clay, which, according to that acute and careful
collector, separates the Cornbrash from the Forest Marble. The beauty
of these specimens, and the admirable state of preservation in which
they are found, forms one of the marvels of the Oolitic fauna ; a
great quantity of fine tests are collected from the gray brashy beds
of the Cornbrash near Chippenham. Mr. Bristow obtained it from the
Cornbrash near Wincanton ; Mr. Pratt from the Forest Marble at Hinton
Abbey ; Mr. Hull found very large specimens in a cream-coloured,
calcareous, semi-indurated, marly bed of the Great Oolite near Burford,
in a quarry not far from the Bird-in-hand Inn. I discovered two
specimens in the Great Oolite limestone of Minchinhampton Common. The
specimens said to have been found in the Inferior Oolite have been
ascertained to be A. Lycetti. The Rev. A. W. Griesbach
collected quinquefid spines of this urchin in the Great Oolite at
Wollaston, Northamptonshire, along with Acrosalenia pustulata.
This species, therefore, belongs to the Bathonian stage, and is a very
characteristic urchin of this great zone of life.
Pl. XV, fig.4
a, b, c, d, e, f, g, h, l, m, d'après Wright