36 PKOE. p. M. DUNCAN AND MB. W. P. SLADEN ON 



poriferous area ; for the expansion of plate 10 extends abactinally 

 so as to crowd out almost the whole of it. 



Plate 9, owing to the same cause, is a demi-plate ; plate 10 is 

 a large tubercle- bearing plate; plates 9 and 10 form a compound 

 plate ; plate 11 is a low primary. The three plates next in 

 succession are a demi-plate, a large primary, and a demi-plate ; 

 and this is a perfect combination-plate and is like those at the 

 ambitus. 



There is much variety in the succession of the plates in the 

 abactinal parts of the ambulacra of different individuals, and 

 indeed hardly two are alike, and the age of the specimen has 

 evidently to do with the diversity. But it is true that a careful 

 examination of a number of specimens shows that there is order 

 in the apparent disorderly arrangement of the plates, and that 

 the perfect condition of the combination-plates of the ambitus is 

 foreshadowed nearer the radial plate. 



In an example of Coelopleurus sindensis, nob., there is a very 

 instructive succession of ambulacral plates near the radial end. 

 In zone " a " there is a succession of primaries from nos. 1 to 8 

 inclusive ; the first five are small aud the sixth is large, narrow to- 

 wards the median line. Plate 7 is a large primary, and is expanded 

 towards the median line both adorally and aboraliy. Plate 8 is 

 also a primary, and, like no. 6, it is contracted at the median line. 

 Now these last three plates form a compound one with the sutures 

 distinct, and there are, of course, three pairs of pores in the 

 multiple plate (Pi. I. fig. 6). The central primary is enlarged 

 towards the median line for a great granule, there being no great 

 tubercles above the ambitus in this species, and the plates above 

 and below are sufii"ering from the pressure of the central primary, 

 but not sufiiciently to destroy their development near the median 

 line and make them demi-plates. A little more pressure would 

 make demi-plates of nos. 6 and 8 *. The next plate to this 

 set is 9, a small demi, a plate once a primary and which has 

 been interfered with by the growth of the triplet immediately 

 above. This demi is a small independent plate. No. 10 is a 

 demi-plate and forms the first member of a large normal combi- 

 nation of a large primary, 11, and its adoral demi, 12. In the 

 instance of this last set, as in all similar to it, the great develop- 



* This combination is of considerable importance in a classiflcatory sense, 

 as one of us will demonstrate shortly in a communication on the Diade- 

 matidse. 



