442 PEOr. p. M. DUNCAN ON THE STETJCTTJEE OE 



well-preserved specimen tlie first great tubercle-bearing compotind 

 plate of ambulacrum III., zone " a," is followed apicaUy by granule- 

 bearing plates, two of whicb clearly form a compound plate ; but tbe 

 third or adoral one seems to have become jammed into the aboralface 

 of the tubercle-bearing plate below (fig. 15). 



Fig. 15 (see p. 452). 



It is evident that there are four plates in this compound one, and 

 that the existence of the small demi-plate is due to the great pres- 

 sure to which it was subjected whilst a primary. 



Hemicidaris granulosa, Wright, a species from the Inferior Oolite, 

 of which there is_ a specimen in the Museum of Practical Geology, 

 Jermyn Street, has the ambital compound plates made up of four 

 primaries. The three placed actinally present the typical Diadema- 

 arrangement, and the aboral plate is a low and broad primary (fig. 16), 



Fig. 16 (see p. 452). 

 XT 



The same arrangement of four plates in a compound one occurs 

 in Hemicidaris Wrighti, Desor, from the Great Oolite. 



In small and immature specimens of Hemicidaris intermedia the 

 plates which bear the great tubercles at the ambitus are made exactly 

 after the fashion of the simplest Pseudodiadema. 



In a specimen of H. jpustidosa, Agass., in the Museum of 

 Practical Geology, the suturing of the plates at the peristome is 

 perfectly visible. The pairs are in triple series, and the plating is 

 like that of the simple Pseudodiadema. 



It is certain that there is no trace of a demi-plate in these last 

 three species. 



It is possible that the existence of a demi-plate in some of the 

 specimens of H. intermedia may be an individual peculiarity ; for 

 in the specimen already noticed with a demi-plate jammed into 

 the aboral part of a compound plate, there is another anomaly. 



The compound plates begin nearer the radial plate than is usual, 

 and the plates of zone " a," ambulacrum III., numbered 31, 32, and 

 33, form a triplet (fig. 17). 



