GENERA. AND GROUPS OF THE ECHINOIDEA. 297 



lacro-inferradial; all others are Transverse. Overlap) or imbrication of 

 the edges of plates may occur when the plates are separated by organic 

 material, although there is no obliquity of the thin edges of the 

 plates. Overlap usually occurs when the plates are in direct contact, 

 and hence obliquity of the edges ; the direction of the plane of suture 

 is then not at right angles with the surface of the test, but more or less 

 aslant. In overlap the edge of a plate orerrides or overlaps the corre- 

 sponding edge of the plate placed actinally or dorsally or at the side. The 

 overlap occurs both in thin and in thick tests and has definite directions. 

 The overlap of the amhulacral plates is adoral or actinal, that is the actinal ' 

 edge of a plate overlaps the abactinal edge of the plate situated below or 

 actinally to it. On the other hand, the overlap of the interradial plates is 

 ahoral or abactinal, and the abactinal edge of a plate overrides the actinal 

 edge of the plate placed dorsally to it. It is understood that the observation 

 must be made of the outside of the test. Overlap may occur at the sides of 

 plates and in the interradia, from the median vertical line outwards. 



Coronal plates are those between the peristomial margir; and the apical 

 system ; p>eristoniial marginal plates surround and form the peristome ; 

 buccal or peristomial plates are in rows or are isolated within the peri- 

 stomial margin in the line of the ambulacra ; periproctal plates are upon 

 the periproctal membrane ; anal plates are close to the anal orifice in the 

 periproctal membrane. 



Interradial plates are simple and variable in number. Amhulacral plates 

 are usually in two vertical rows, but there may be more ; the plates are 

 either simple or compound ; and in the first case they are in the great 

 majority of instances ^>'2Hiarzes, that is they extend from the outer edge 

 of an ambulacrum to the median suture of the area. When compound 

 they are composed of two or several components, all of which are joined 

 by sutures and form a geometrical plate ; some components are primaries, 

 that is extend from the side of the ambulacrum to the median line ; 

 others are demi-plates and do not reach the median line. The direc- 

 tion of the lines of suture of the components of a compound plate varies in 

 difierent families. 



A plate is said to be occluded when it reaches the median line, but does 

 not touch the ambulacro-interradial vertical suture. The plate may be 

 isolated, and then it is shut out from the median line as well as from the 

 edge of the ambulacrum. Each plate, whether simple or forming part of a 

 compound plate, or a solitary demi or occluded plate, is perforated either 

 by two canals, and shows externally a pair of pores, or by a single canal, 

 or by several, each having one pore externally. (See Tentacles.) 



Tubercles are primaries when fully developed and larger than others, 

 which are "Secondaries." They have a base, the "5oss," which carries 

 the " Mamelon ; " the upper part of the Boss may have crenulation, or be 

 crenulated (more or less vertical ridges and grooves all round the upper 

 part) or not, and then it is said to be plain ; the mamelon may have 

 a cii'cular foramen leading downwards from the apex for a slight distance, 

 and is then "perforated," or the perforation may be absent, and the 

 mamelon is " imperforate." A plain or a sunken space may surround the 

 base of the Boss of primary tubercles, which is called the " Scrobicule " or 



