THE FAMILY AEBACIAD^. 45 



first tubercle-bearing compound plate is formed by the three plates 

 at the top of the ambulacrum, of which the first is, as usual, a 

 primary, the second is a demi, and the third, which, seen from the 

 outside of the test, is closely connected with the tubercle, forms 

 a considerable expansion aborally and beyond the median line 

 of the ambulacrum. The adoral expansion is almost nil. The 

 second plate has evidently commenced as a small primary, and 

 has been prevented growing towards the median line by the 

 peculiar expansive development of the third, which is a primary, 

 plate. Then come two primaries in succession ; they have no 

 expansions, are low in vertical measurement, and are almost 

 without granules on the outside of the test. A triplet follows 

 corresponding to a tubercle-bearing plate ; but the arrangement 

 is not yet that of the compound plates at the ambitus. There 

 is a demi-plate 6, followed adorally by a large primary 7, with the 

 bulk of the tubercle upon it, and the plate extends to the median 

 suture of the ambulacrum, the whole of the inner part of the com- 

 pound plate belonging to it. No suture crosses this plate from the 

 interradial edge. The next plate, 8, completes the triplet, and it ia 

 a low primary that reaches the median line with a narrow termi- 

 nation. It is to be distinguished externally by a minute tubercle 

 being placed on it near the centre of the space between the second 

 and third tubercles. The next triplet consists of plates arranged 

 as is the case lower down, and there is an aboral and an adoral 

 demi-plate with au intermediate expanded primary, which mainly 

 carries the third tubercle from the apex. After this plate there 

 is the same arrangement found as at the ambitus. 



Now the triplets extend much over the vertical median line of 

 the ambulacra ; and the result is to prevent the development of 

 tubercles side by side near the apex, where there is much pressing 

 and crowding of plates. Hence, on examining the opposite zone 

 of the ambulacrum, zone "a," from within, it is found that the 

 following is the succession of the plates : — 



Plate 1, a small primary. 



2, a small primary. 



3, a small primary. 



4, a demi-plate. 



5, a large primary. 



6, a low primary. 



7, a demi-plate. 



8, a large primary'. 

 9j a demi-plate. 



