114 Prof. P. M. Duncan on the 



the ])late, some small ones being amongst the pairs of pores, 

 which are large and in peripodia and separated by a narrow- 

 process. On looking at the inside of the corresponding part 

 of the test (fig. 7) it is impossible to recognize the ex- 

 ternal arrangement. The pairs of pores are in a very slightly 

 curved vertical series, and the adoral pair of perforations (a) 

 is not much or at all out of the direction of the other two 

 pairs. But the obliquity of the canals of the adoral pair is 

 great (fig. 8), and in all cases there is more plate-structure 

 between the pairs of pores and the ambulacro-iuterradial 

 suture than is visible on the outside. The pores are quite 

 1 millim. to 1"5 millira. apart, on the inside of the test. 



The test is very thick, and the ambulacro-interradial sutures 

 are quite obliterated, and fracture will most certainly not occur 

 along that line. The sutures between the compound plates 

 are slightly depressed or broadly grooved on the free outer 

 surface of the test, and the pits are exceedingly shallow. 



Taking a plate at the ambitus which is normal, and they 

 are rare, and applying benzule, a line of suture is seen to 

 pass below the upper peripodium of the triplet with a down- 

 ward curve across the plate, to reach the median sutural edge 

 close to the aboral median angle. This suture marks the 

 adoral boundary of the upper component of the plate, and 

 it is a primary (fig. 6, c). 



A. line passes adorally to the middle peripodium [h) of the 

 three, and curves with a slant upwards and joins the other 

 suture at no great distance towards the median line. This 

 line is the adoral suture of the median plate of the compound, 

 and it is a small demi-plate (fig. 6, h) . 



The line just mentioned nearly touches the aboral edge of 

 the lowest or adoral peripodium (a) of the innermost pair of 

 pores, and they are placed in a primary plate, which carries 

 the ornamentation of the greater part of the combination 

 (fig. 6, a). 



An inner view of this plate (fig. 7) shows a simple curved 

 series of pores, the width apart of the pores of a pair being 

 much greater than in a peripodium. The path of the sutures 

 is very distinct and is very like that of Salmacis. The ob- 

 liquity of one of the canals of the adoral pair is shown in 

 fig. 8. 



Near the apical system the compound ambulacral plates are 

 narrower than at the ambitus, but are made much upon the 

 same plan ; but a little lower down very remarkable differ- 

 ences are seen on the inside of the test. 



A compound plate will be seen not far from the apex, 

 within, and its component plates are all primaries (fig. 9, a?), 



