THE FAMILY AEBACIAD^. 35 



poriferous part, does not extend to tlie median line, becomes a 

 demi-plate, and rests against a large plate with which it will 

 eventually be sutured as j)art; of a triplet. As the increase of 

 small primaries just below the radial plate is constant up to a 

 certain age of the test, it follows that young forms have fewer 

 small primaries than older and more conically-topped tests. 



The order of the plates in a medium-sized individual appears 

 to be as follows. The ambulacrum chosen for description is 

 No. III. of Loven's nomenclature (the ambulacrum immediately 

 to the left of the madreporite, and its zones, are " a " and " 6," 

 PI. I. fig. 5). 



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

 the radial plate actinally, it is found that there is the following 

 succession (PI. I. fig. 5) : — Plates 1, 2, 3, 4 are small pri- 

 maries gradually increasing in size. The plate 5, also a primary, 

 is pushed, as it were, aborally, by the aboral expansion of the 

 seventh plate, and just escapes being a demi-plate, for it only 

 touches the median suture by a narrow point. Plate 6 is a 

 demi-plate, separated from the median sutural line by the large 

 tubercle-bearing plate 7 ; this small demi-plate (6) forms the 

 shoulder of the tubercle, plate 7. Plate 8 is a low primary, and 

 so is plate 9. Plate 10 is a primary which, were there a slightly 

 greater expansion of the next great tubercle-beariug plate, 12, 

 than there really is, would, like its successor plate 11, become a 

 demi-plate. Plate 11 is a small demi-plate and forms the aboral 

 demi-plate of a triple combiiiatiou-plate. Plate 12 is the central 

 primary of this compound tubercle-bearing plate, and plate 13 is 

 a demi-plate. This combination of plates 11, 12, and 13 some- 

 what resembles those found lower down. 



In zone "5" the first three plates near the radial plate are 

 small primaries. Plate 4 is a demi-plate, and it is crowded out 

 from the median line by the abactinal expansion of a small 

 tubercle-bearing plate, 5. These two plates make up a compound 

 plate . Plate 6 is a small primary with the part in the inter- 

 poriferous area made angular and low. 



This is the result of the pressure and crowding incident on the 

 actinal expansion of the tubercle-bearing plate 5. The crowding 

 is not sufficient to make the plate 6 a demi-plate, but it nearly 

 does so. Plate 7 is a low primary with a granular ornamen- 

 tation. 



Plate 8 is a small primary with a low angular part in the inter= 



3* 



