Dr. T. Wright on the Cidaridse of the Oolites. 243 



one is larger and more prominent than the others, presenting a 

 spongy porous surface, and called the madreporiform plate ; it 

 is placed opposite the ambulacra, which is the analogue of the an- 

 terior area in the Spatangus, and occupies therefore the posterior 

 border of the apical disc, affording thereby a key for ascertaining 

 the antero-posterior diameter of the body ; the other four ovarial 

 plates are disposed in pairs before the single madreporiform plate. 

 The polygonal plates of both arese are arranged in double ver- 

 tical rows, two columns of ambulacral plates alternating with 

 two columns of interambulacral plates ; the plates of each pair 

 are united by a zigzag suture formed by the re-entrant angles 

 of the plates ; the plates of the ambulacra are united to those 

 of the interambulacra by minutely serrated edges. The porife- 

 rous zones have small plates, the sutures of which cut through 

 the centre of the holes, by which arrangement the enlargement 

 of the foramina with the growth of the test is provided for. 



The surface of the test is covered with tubercles for supporting 

 spines ; these are of two kinds, the principal and the miliary 

 tubercles. The principal tubercles are in general raised on mam- 

 millated eminences with or without crenulations at their summit, 

 and arranged in vertical rows on the sides of the arese between the 

 mouth and the anus. The miliary tubercles are much smaller 

 and more numerous ; they are not disposed with the same regu- 

 larity, but are frequently scattered on the surface of the plates, or 

 disposed in circles around the bases of the principal tubercles. 



Each tubercle supports a spine, the size of which corresponds 

 with that of its tubercle ; the spines are composed of three distinct 

 parts, the stem, the neck, and the articular head. The stem is 

 more or less elongated and of various forms ; the head is sur- 

 rounded by a raised ridge, and has a concave excavation for its 

 articulation with the tubercle ; the head is separated from the 

 stem by a smooth neck, the extent of which varies in the differ- 

 ent species. The spines present very numerous modifications of 

 size, form and sculpture, which are closely connected with specific 

 distinctions ; some are elongated, cylindrical, fusiform, or subu- 

 late; others are compressed, spatuliform, or triangular ; whilst 

 others, on the contrary, are expanded, pyriform or claviform. 



The surface of the spines is smooth, striated, or furnished with 

 granules, prickles, or other asperities disposed in regular order or 

 scattered at hazard over the stem. The same individual has its 

 test occupied with different kinds of spines ; hence the great im- 

 portance of obtaining these appendages in connection with the 

 test. 



We have made the following estimate of the number of sepa- 

 rate pieces which enter into the composition of the test of Echi- 

 nus sphara : — 



16* 



