246 ACROSALENIA. 



AcRosALENiA WiLTONii, Wright. PI. XVI, fig. 3 a, b, c, d, e. 



AcBOSALENiA WiLTONii. Wright, Annals and Magazine of Natural History, 2d series, 



vol. ix, p. 83, pi. 3, fig. 4 a — e. 



— — Forbes, in Morris's Catalogue of British Fossils, 2d ed., p. 70. 



— Lamarckii. Desor, Synopsis des Echinides Fossiles, p. 141. 



Test hemispherical, depressed, sometimes elevated ; sides always tumid ; ambulacral 

 areas narrow, straight, with two rows of small perforated tubercles on the margins, set 

 wide apart, and a miliary zone of three rows of fine granules between the marginal tuber- 

 cles ; inter-ambulacral areas four times the width of the ambulacral ; ten tubercles in each 

 row — of these the three middle pairs only are developed, the basal are small, and those 

 at the upper surface rudimentary ; miliary zone wide, composed of four rows of small 

 granules in the middle, and' six rows above; apical disc convex and prominent; sur-anal 

 plate formed of two large and five small pieces ; basal angle obtuse, from the tumidity of 

 the sides ; base concave ; mouth opening small ; peristome unequally decagonal. 



DimensionH. — Height, half an inch ; transverse diameter, one inch. 



Description. — This beautiful little urchin has aknost always a more or less elevated 

 hemispherical test, which is rarely as much depressed as in fig. 8 c ; the sides are tumid, 

 and the base is concave ; the ambulacral areas preserve a very uniform width throughout ; 

 two rows of small perforated tubercles placed wide apart occupy the sides of the area 

 (fig. 3 ^), with from eighteen to twenty in each row; the eight basal pairs are larger, and 

 the lateral and dorsal pairs smaller, they are arranged closer together, and gradually diminish 

 in size until they become quite microscopic (fig. ^ d) ; two or three rows of fine granules 

 (fig. 3 d) form a miUary zone, which sends lateral branches to divide the small marginal 

 tubercles. 



The poriferous zones are narrow and undulated, the pores are unigeminal, except near 

 the base, where they are trigeminal (fig. 3 h) ; opposite each of the large lateral plates 

 there are eight pairs of pores (fig. 3 f/). 



The inter-ambulacral areas are four times as wide as the ambulacral ; they have ten 

 pairs of tubercles ; the three basal pairs are small (fig. 3 h), gradually increasing in size 

 from the peristome upwards, to blend in with the three lateral pairs, which are the largest 

 (fig. % c, d) \ the eighth pair are suddenly smaller than the seventh pair, and the ninth and 

 tenth pairs are quite rudimentary (fig. 3 a) -. the miliary zone is wide (fig. 3 c), and occupied 

 by four or five rows of small close-set granules on the sides, increasing to six or seven rows 

 on the upper surface; between the areolas and the poriferous zones there are likewise two 



