THE AMBULACKA OF POSSIL ECHINOIDEA. 423 



broad primaries near the radial plate, and extending far towards 

 the ambitus, and each plate is united by a separable suture to 

 the plates situated apically and actinally. At the ambitus three 

 primaries are seen to form a compound plate geometrical in figure, 

 and their separability is at an end. There are triple plates of 

 this character down to the peristome. Taking a compound plate 

 at the ambitus (fig. 2) as the example, it appears that it is 



Pig. 2 (see p. 451). 



broader than high, and that the pairs of pores, three in number, 

 are either straight or in a very slight curve. Two lines of suture 

 cross the compound plate, and they indicate the edges of the 

 primaries which have united to form it. One line is between the 

 aborally situate plate and the middle one of the combination, and 

 it passes from the adoral pore of the aboral pair inwards to 

 cross the base of the tubercle apically to the mamelon, and thence 

 it passes on to the median suture of the compound plate or, 

 as it is called, the vertical suture or edge. This line is very 

 faintly curved with the convexity towards the tubercle, or it 

 may be straight and cross the fiank of the boss. The other sutural 

 line is between the middle plate and the adoral plate, and it is in 

 contact with the adoral pore of the second or middle pair ; thence 

 it crosses the adoral flank of the boss actinally to the mamelon 

 and reaches the vertical suture with or without a curve. The 

 aboral plate of the combination is a low and broad primary which 

 includes a small portion of the boss, and the adoral plate is 

 of the same configuration as the other. The middle plate is rather 

 the largest, and is a well-formed primary which carries much of the 

 boss of the tubercle and all the mamelon. 



The sutural lines between the. plates constituting the compound 

 plates are in some instances decidedly curved, with the convexity 

 placed towards the mamelon ; but this condition is replaced nearer 

 the peristome by quite straight lines. The formation of the almost 

 rectangular primaries in the combination resembles a simple appo- 

 sition of Cidaris-like plates, and it is interesting to notice this 

 primitive type merging gradually into the fully developed one, in 

 which, as in the ancient and modern Diadematoids, the curving of the 

 sutural lines is coincident with an enlargement of the middle plate 

 towards the median line at the expense of the plates above and 

 below, these plates then departing from the shape of those of the 

 Cidaridse. 



Hemipedina Boweebankii, Wright. 



There are some fine examples of this species in the Museum of the 



2g2 



