204 PROF. P. MARTIN DUNCAN ON THE 



Glypeasfer, and Laganum as types, the nature of the relics of 

 the disconnected perignathic girdle can be appreciated. 



Clypeaster {JEchinanthus, A. Ag.) rosaceus, Linn. — When the 

 abactinal part of the test is removed and the jaws also, the inner 

 surface of the actinal part is seen, and the five ambulacra are 

 noticed to be broad at the peristomial edge and each commences 

 there with a plate on either side of the ambulacral median line. 

 These plates are perforated by a pair of large pores close to the 

 edge, and all the rest is furrowed from side to side and penetrated 

 by a multitude o£ very small pores (fig. 53). The side sutures of 

 these ambulacral plates (the ambulacro-iuterradial) are visible at a 

 short distance from the peristomial edge but not up to it. The 

 interradial plates at the peristomial edge are not one half of the 

 breadth of an ambulacrum there. Benzene shows that there is 

 but one plate in the interradium, whilst there are two to an 

 ambulacrum. See also Loven, Etudes, pi. xlvii.* Moreover it is 

 seen that one of the processes of the incomplete and very dis- 

 connected perignathic (or rather infragnathic in position) girdle 

 has its narrow base limited on one side by the lateral suture of 

 the interradial plate 1, and that this plate is crushed in between 

 the process and the one on th.e other side (fig. 54). It is evident 

 that the interradial plate no. 1 is narrow and yet long, from the 

 peristomial edge towards the circumference, or outwards ; and it 

 is seen that the second pair of ambulacral plates (plates zone «2 

 and zone h 2) are so broad that they extend right into where there 

 should be interradial plates nos. 2, and unite by suture with the 

 second plates of the next ambulacra (fig. 53). The result is 

 that the plate 1 of the interradia is separated circumferentially 

 from the second pair of interradial plates, which are found 

 further outwards. The interradial plate 1 is not covered by a 

 process, but it is between two processes. A process arises from 

 a narrow but long base (fig. 53, p), which is in that part of an 

 ambulacral plate where the numerous pores seem to end without 

 coming up to tbe ambulacro-iuterradial suture. The spot is the 

 posterior or circumferential and outer corner of the plate, and it 

 is of course far from the median ambulacral line (fig. 58). 



Careful amplification shows that the pores are continued in 

 small pairs, placed rather wide apart, upon tlie flank of the process 



* Loven, 'Etudes,' gives admirable dissections of the plates of the Olypeas- 

 troids. He does not descrioe the processes, however. 



