Thomas Wright

 in T. WRIGHT (1857-1878) Monograph on the British Bossil Echinodermata of the Oolitic Formations

Vol.I - The Echinoidea, pages 238-240.

ACROSALENIA SPINOSA, Agassiz.  Pl. XVII, fig. 3 a, b, c, d, e, f.

Test sub-pentagonal, depressed ; ambulacra straight, prominent, with two marginal rows of small spaced-out tubercles ten to twelve in each row ; inter-ambulacra with eight to nine primary tubercles in each row, large at the circumference, gradually diminishing towards the peristome, suddenly so on the upper surface ; miliary zone narrow at the circumference, wide and naked jn the middle at the upper surface; apjcal disc large, sur-anal plate single ; vent large, excentral ; mouth large ; peristome decagonal, lobes nearly equal ; notches slightly incised.

Dimensions.- Height, nearly four tenths of an inch ; transverse diameter, seven tenths of an inch.

Description.-This beautiful little urchin is so admirably preserved in the Cornbrash of Wiltshire, that it forms one of the most characteristic fossils of that formation ; the test is small, sometimes circular, oftener sub-pentagonal, and always much depressed (fig.3 c).

The ambulacral areas are narrow and prominent (fig. 3 a, b) ; as the poriferous zones are wide, for so small an urchin, the ambulacra appear isolated, and the upper surface of the test has a radiated appearance, which is increased by the size and smoothness of the apical disc (fig. 3 a) ; the ambulacra converge in straight lines from the base to the disc ; they are furnished with two rows of small tubercles, which, although microscopie in size, have crenulated bosses and perforated summits; the tubercles are disposed with great regularity on the borders of the areas, at a moderate distance apart (fig. 3 c, d), so that there are only from ten to twelve in each row; in the middle of the are a a double row of granules separates the tubercles, and lateral rows encircle the areolas, which are only open on the sides (fig. 3 d). The poriferous zones are wide and straight; the pores are large, and disposed obliquely in single pairs (fig. 3 d), forming a rectilinear file on each side of the ambulacral segments ; there are from five to six pairs of pores opposite each large tubercular plate.

The inter-ambulacral areas are three times the width of the ambulacral ; the two rows of tubercles, about eight in each row, are nearly equidistant throughout (fig. 3 a); the two basal pairs are small (fig. 3 b), the three equatorial pairs large (fig. 3 c), and the three upper pairs suddenly diminish in size and become dwarfed as they approach the disc (fig. 3 a, c) ; two or three pairs of tubercles at the equator have very large bosses (fig. 3 d), whilst all the others are of moderate size ; they have narrow elliptical areolas, of which the larger are confluent (fig. 3 d) ; the miliary zone consists of two rows of granules, which form scrobicular circles around the areolas (fig. 3 d) ; as the rows diverge above, a small triangular space on the upper part of the segment is left naked (fig. 3 a) ; the areolas are likewise separated from the poriferous zones by a single row of granules (fig. 3 d).

The apical disc is large and pentagonal, about two fifths the diameter of the test (fig.3 a) ; the anterior and posterior pair of genital plates are nearly alike in size and form (fig. 3 f) ; the oviductal holes are perforated near the external third of the plates ; the posterior single plate projects a little further into its corresponding segment than the others, it is largely excavated for the vent, of which it forms the posterior wall (fig. 3 f) ; the sur-anal plate is central, single, and pentagonal, and forms the anterior wall of the vent (fig. 3 f), its sides being formed by the posterior pair of ocular plates, which are much larger than the three others (fig. 3 f) ; the madreporiform body is very small, and occupies as usual the right anterior genital plate ; a few small granules form central clusters on the pairs of genitals, and similar granules dot the surface of the small oculars (fig. 3 f).

The mouth opening is nearly half the diameter of the test (fig. 3 b) ; the peristome is decagonal, and divided by small notches into nearly equal-sized lobes ; the incisions are not deep, and the margin is reflected over all the angles (fig. 3 f).

Affinities and differences.-This species is distinguished by its small size and depressed test, the extreme regularity of its tubercles, the radiated appearance of the upper surface, the apical disc having the sur-anal plate single, and the vent of a triangular form; these characters serve to distinguish it from all its other English congeners. It resembles A. aspera in many of its general characters, but that species has undulated ambulacral areas with close-set tubercles and the sur-anal plate composed of two pieces ; it is so entirely distinct from all other forms of the genus at present known, that it is unnecessary to pursue a comparison with them.

Locality and Straligraphical position.-I have collected two specimens of this urchin in the Pea grit, Inferior Oolite, at Crickley Hill, and in the yellow clay resting on the Stonesfield Slate at Sevenhampton, with Anabacia orbulites, Pecten vagans, Ostrea acuminata, and other Great Oolite fossils. l have likewise found it in the Great Oolite at Sham Castle, near Bath. Mr. Lycett has collected it from the Great Oolite of Minchinhampton Common, where it is small in size and rare. It is abundant in the Forest Marble and Cornbrash near Chippenham, with Avicula echinata, Sow., whence my finest specimens were collected by Mr. William Buy. It is found, likewise, in the Forest Marble and Cornbrash near Cirencester ; indeed, it may be considered an abundant fossil in the English Cornbrash, but l have seen no specimens at all equal in their preservation and beauty to those found in Wiltshire.

ln Switzerland it was collected by M. Strohmeyer from the marls containing Ostrea acuminata, in the canton of Soleure. ln France it has been found in the Great Oolite of Ranville, Calvados, by Professor Deslongchamps, and in the upper beds of the Bathonien étage in the environs of Châtel-Censoir, by M. Cotteau.