136 Dr. T. Wright on the Cidaridae of the Oolites. 



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 sphcera : — 



Interambulacral areae 32 plates in each column 32 X 2 X 5= 320 plates. 

 Ambulacral areaj 80 do. do. 80x2x5=800 do. 



Poriferous avenues 160 do. do. 160x2x5=1600 do. 



Apical disc 10 plates 10 do. 



Each interambulacral plate supports 10 tubercles 320 X 10=3200 tubercles. 



Each tubercle supports a moveable spine 3200 spines. 



Each ambulacral plate supports 2 tubercles ... 800 X 2 1600 tubercles. 



Each tubercle supports a moveable spine 1600 spines. 



There are 70 rows of holes in each avenue, 



and in each row these six holes are disposed 



jn pairs obliquely 70x6x 10=4200 foramina. 



