174 MR. H. L. HAWKINS ON THE 



(iii.) Finally, tlie pores become almost circular in shape and well 

 spaced out anteriorly, progressively decreasing in size until the 

 fasciole is reached. 



No sutui'es can be seen on the outer surface of the test in the 

 two adapical sections of the area, but some indication of them, 

 resulting from the inflation of the plates, can be seen from within. 

 By applying the method above described, they can be rendered 

 clearly visible from the outer view, and the following features 

 become manifest. 



" Plate-crushing " is restricted to sections i. and ii. of the area. 

 In those sections the plates are very low, esjiecially in i. and the 

 anterior part of ii. In iii. they rapidly increase in height. 



In section i. there is usually a large preponderance of primary 

 plates, and the type of " plate-crushing" found is that charac- 

 teristic of most Echinoids, namely, the production of demi-plates 

 (in contact with the adradial, but not reaching the perradial, 

 sviture). The demi-plates are developed quite irregularly. 



In section ii., where the ambulaci'um attains its greatest width, 

 the structure is far more complex. The plates in some parts 

 become very low, often being mere strips of calcite. Demi-jolates, 

 occluded plates (in contact with the perradial, but not reaching 

 the adradial suture), and even " klasma "-plates (not reaching 

 either vertical suture) are developed, and a consequent displace- 

 ment of the pores occurs. In some rare instances the pores have 

 atrophied in occluded plates. The anterior part of this section 

 is built of relatively high plates, and the pores are of veiy large 

 size. The " plate-crushing " here results in the formation of 

 occluded plates, which, however, almost reach the adradial suture. 



In all of the specimens (over 150) examined, individual pecu- 

 liarities and iiTegularities, though frequent, are insufficient to 

 mask the general pattern. The differences met with seem, 

 moreover, to conform to two general conditions. These can be 

 roughly indicated as being («) the i-elative width of the area, and 

 (b) the length of the area affected, measured in proportion to the 

 size of the specimen. 



(a) There is a great vai'iation in the degree of lateral expansion 

 attained by the ambulacrum in its middle section. This character 

 lias no connection with different age, as specimens of practically 

 the same size may show the extremes of difference. It seems an 

 invariable rule that in the narrower areas the development of 

 occluded and "klasma "-plates is greater than in the broader ones. 

 A comparison of text-figs. 39 and 40 will make this clear. The 

 development of these small plates can thus be ascribed directly to 

 lateral compression. 



(6) Individuals of the same size have approximately the same 

 number of ambulacrals (about 69 in large specimens) between 

 ocular III. and the fasciole. The fasciole crosses the ambulacrvmi 

 from the 8th or 9th interambulacral plates on each side irre- 

 spective of the size of the specimen. The position of the apical 

 system is variable, being sometimes almost central, and at others 



