158 Dr. T. Wright on the Cidaridse of the Oolites. 



The two specimens which have preserved their form and served 

 for the foregoing description were only obtained within the last 

 few days ; all those previously collected having been too much 

 injured to serve for minute observation. 



I dedicate this species to my friend John Lycett, Esq., one of 

 the learned authors of a monograph of the Mollusca from the 

 Great Oolite. 



Acrosalenia spinosa, Agassiz. PL V. fig. 3 a, b, c, d. 

 Acrosalenia spinosa, Agassiz, Echin. de la Suisse, 2nd part, t. 18. 

 fig. 1-5. p. 39; Cotteau, Echin. Foss. du Departement de l'Yonne, 

 t. 3. fig. 6-11. 

 Test subpentagonal, depressed ; a double row of small tubercles 

 occupies the ambulacra, and a double range of large mammil- 

 lated tubercles the interambulacral areas ; the ovarial disc is 

 large, the madreporiform plate rudimentary, the anal opening 

 behind the supra-anal plate ; mouth decagonal, margin deeply 

 incised. 



Height T 3 G ths of an inch, transverse diameter ^fths of an inch. 

 Description. — The test of this beautiful little Urchin has a sub- 

 pentagonal form arising from the convexity of the ambulacral arese, 

 which converge in straight lines from the base to the summit, 

 and are furnished with two ranges of from ten to twelve very small 

 tubercles, which, although microscopic, are nevertheless mam- 

 millated and perforated. The intertubercular spaces are covered 

 with small granules which form circles around the tubercles. 

 The pores are disposed obliquely in simple pairs, forming a single 

 rectilineal file on each side of the arese. The interambulacral 

 areas are twice the width of the ambulacral, and ornamented with 

 a double range of primary tubercles, eight in each range. The two 

 inferior tubercles are small, the two or three succeeding ones are 

 very large, whilst those on the upper part of the test suddenly di- 

 minish in size and gradually become dwarfed as they approach the 

 anal disc : they are all crenulated and perforated. The primary 

 tubercles occupying the equator of the test are seated upon large 

 prominent mam miliary eminences, surrounded by deeply grooved 

 elliptical areolae, and encircled by a wreath of small granules. 

 The intertubercular surface on the upper part of the test is 

 studded with very fine granules. The apical rosette, formed of 

 ovarial, ocular, and sur-anal plates, is admirably preserved in the 

 specimens before us ; it is large and of a pentagonal form (fig. 3 d). 

 The two anterior pairs of ovarial plates are nearly of the same size, 

 the posterior pair being notched to form the basal angles of the 

 triangular anal opening \ the sur-anal plate occupying the centre 

 of the rosette is small, single and pentagonal ; the single or madre- 

 poriform plate is rudimentary, to make space for the apex of the 



