236 Prof. P. M. Duncan and Mr. W. P. Sladen on the 



on either side of it, and the plastron is on a lower level than 

 the rest of the actinal surface. The first plate (No. /) of this 

 interradium (No. 5) is much the broadest of all the correspon- 

 ding plates around the mouth, and it is " cheese-knife " in 

 shape — that is, convex at one end and broad, but narrowing 

 behind to an almost straight handle. The convex part nearly 

 forms the whole of the posterior lip of the peristome, but parts 

 of it are made up of ambulacral plates, and these indent the 

 sides of this first interradial plate and produce the peculiar 

 shape of its sides. Plates a 2 and h 2 oi interradium 5 are very 

 long ; they unite along the median line by a straight suture, 

 and are in contact with plate 1 by means of a narrow forwardly 

 curving suture, the breadth of the suture of " a " and " i" 

 being equal. The plate a 2 is longer than plate h 2 ; both are 

 narrow, widest in the middle and becoming narrower back- 

 wards. The smaller plate h 2 unites posteriorly with plate 

 h 3, and the long plate a 2 is in contact posteriorly with 

 plate a 3, and also by its proper right posterior suture with 

 plate b 3. The plates a 3 and b 3 are in contact by the left 

 posterior suture of b 3, and both of these plates are sutured to 

 plates «/^ and 5 4 respectively. The plates a J^ and b 4. are 

 larger than plates 3. 



The following is the arrangement of the ambulacral plates 

 in relation to the several interradial plates of interradium 5. 

 The narrow part of plate / is in contact with the large ambu- 

 lacral plate amb. I. a /, and the following ambulacral 

 plate a 2 and one half of plate a 3 are also boundaries of the 

 large interradial plate 1. The remaining half of ambulacral 

 plate a 3 and all of a 4 and part of a 5 limit the interradial 

 plate b 2. Now it is imjjortant to obsei've that the posterior half 

 of the ambulacral plate a 5 is in contact by suture xcith the inter- 

 radial plate b 3, but not with a 3, for that is on the fwther 

 side ofb3] moreover the ambulacral plate a 5 does not encroach 

 towards the median line. 



This description refers to the usual condition of the inter- 

 radium, but there are occasional departures from the normal, 

 which, however, do not affect the argument. Thus in one 

 instance the interradial plate a 2 is crossed by a suture on a 

 level with the posterior suture of the ambulacral plate b 2 oi 

 zone v., but the arrangements of the interradial plates further 

 back is the same as in the type. 



With regard to the width of the ambulacral plates it is 

 evident that as a whole they are narrow ; the plates a /, b 1, 

 and a 2, 6 i2 of ambulacrum I. are broader than the following 

 plates 3 and ^^ of a and b. Plates a 5 and b 5 are broader than 

 those just noticed, are })]aced side ])y side as is usual in 



