THE AMBULACRA OF FOSSIL ECHINOIDEA. 445 



aboral edge is transverse, and the actinal is oblique from the median 

 upwards and outwards. The pair of pores is decidedly oblique, and 

 the aboral pore is on a vertical line with the adoral pore of the 

 second plate. 



Plate four is a primary with a large expansion in the poriferous 

 part and a very low and almost linear iuterporiferous area. 



Its pair of pores are nearer the interradium than those of plate 

 three. Plate five is a primary, its shape is irregularly rectangular, 

 and it is larger than any of the plates noticed ; the pair of pores can 

 hardly be said to be one of the inner set, and there is a small 

 tubercle close to the median line. 



Pollowing the rule which has been observed in recent forms, this 

 tubercle, being bound to grow in all directions, prevented the adoral 

 growth of the plate immediately above. 



Plate six is low externally and higher somewhat towards the 

 median line. Its pair of pores are in a peripodium, and they belong 

 to the inner row. Plate seven, also a primary, is largest close to the 

 interradial edge, where the pair of pores are, and it is almost linear 

 towards the median line. It is decidedly broader than the other 

 plates hitherto noticed ; and indeed every plate in this region becomes 

 broader as the distance from the radial plate is increased. 



This plate is one of the outer set. 



Plate eight is very low externally, swells at the peripodium, and 

 is higher at the median line ; its pair of pores belong to the inner 

 series. Plates ten, twelve, and fourteen belong to the same series 

 as plate eight, and are formed after the same plan ; they are all pri- 

 maries, and have low and almost linear outer parts ; the pores belong 

 to the inner set and the jDlates are highest at the median line. On the 

 other hand the plates nine, eleven, thirteen, and fifteen are upon the 

 same type as plate seven, and belong to the outer series, with low 

 a7id linear portions near the median line and enlarged poriferous 

 zones. 



In a young specimen the alternation of diff'erent-sized plates, all 

 of which are primaries, extends at least to the twenty-fourth plate. 

 Then the primaries become combined into a compound plate, some 

 being blocked out, however, from the median line and becoming 

 demi-plates. This condition is seen in adult as well as in young 

 specimens, and just where the bigeminal pores begin to diminish 

 and to be replaced by simple pairs. 



The first compound plate is a tubercle-bearing one in the specimen 

 under examination ; there are four pairs of pores belonging to 

 it and they are in double series (fig. 19). (The upper two pairs do 

 not belong to the plate.) 



Fig. 19 (see p. 452). 



