292 THE FOSSIL ECHINOIDEA 



the inner sutures is nearly vertical, and the upper and lower margins of the demi- 

 plates are nearly horizontal, instead of converging more or less. The plates become 

 small as they approach the margin of the peristome, are slightly inverted, and the 

 last is a small demi-plate the adoral pore of which is incomplete and opens at the 

 peristome. 



The outer part of the smaller poriferous plates near the peristome extends suffi- 

 ciently to form part of the surface of the prolongation of the small branchial cut which 

 may be termed a " tag." The termination of the ambulacra at the peristome is slightly 

 incurved and reaches slightly nearer the centre of the actinal space than the ends of 

 the interradia, which are straight and narrow. There is a concave edge to the inter- 

 poriferous area at the peristome, and immediately a little transverse projection of the 

 test is passed, towards the ambitus, there appears a large central pit, and it is evidently 

 on the median suture between two tubercle-bearing plates. In well-preserved speci- 

 mens of C. Forbesi the pit is as large as the mamelon of the tubercles close by, and 

 whilst the base of the pit, or rather its actinal edge, is transverse and straight, the 

 upper or aboral edge is arched. There are usually five similar pits in each ambu- 

 lacrum and they are large, shallow, arched when well preserved, and they are placed 

 at the junctions of the median with the lateral suture. 



The arrangement of the plates of the ambulacra is readily perceived at the ambitus, 

 but it becomes complicated near the radial end, where there is considerable variation 

 in the succession of small primaries, demi-plates, low primaries, and an occasional large 

 primary. The arrangement of the plates, in this part, foreshadows and finally merges 

 into the triple compound arrangement seen at the ambitus. 



In a well-preserved specimen of nearly adult size, the following succession of plates 

 will be found in ambulacrum III. : — 



Taking zone "a" and noticing the nature of the plates from the radial plate 

 actinally, it is found (Fig. 7, PI. XL VI) that there is the following succession : 



Plates 1, 2, 3, 4 are sndall low primaries gradually increasing in size. Plate 4, 

 a primary, is pushed as it were aborally by the aboral expansion of plate 7, and just 

 escapes being a demi-plate, for it only touches the median line by a narrow point. Plate 

 6 is a demi-plate, separated from the median line by the large tubercle-bearing 

 primary plate 7, and it forms the shoulder of the tubercle and is very small. Plate 7, 

 a tubercle-bearing primary. Plate 8 is a low primary, and so is plate 9. Plate 10 is a 

 primary which, were there a greater expansion of the tubercle-bearing plate 12, would, 

 like its successor 11, become a demi-plate. Plate 11 is a small demi, forming the aboral 

 demi-plate of a triple combination plate made up of plates 11, 12, and 13. Plate 12 

 is the tubercle-bearing primary, and 13 is a large demi-plate. After this there comes 

 the normal compound plate of the ambitus. 



In zone " b " the three plates nearest the radial plate are small primaries ; the plate 

 4 is crowded out from the median line by the abactinal expansion of a small tubercle- 

 bearing primary 5, and thus plate 4 becomes a demi-plate and is combined with plate 5. 

 Plate 6 is a primary with a small angular interporiferous part in consequence of 

 the downward extension of plate 5 occupying much space. This angular primary is 



