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



by the ambulacra; on the other hand, the margins are formed 

 by the very broad ends of the first plates of the interradia 4 

 and 1. The most conspicuous feature of this part of the test 

 is this preponderance in size of the marginal interradial plates 

 over the marginal ends of the ambulacral plates. 



Continuing the description of the interradium No. 5, it is 

 only necessary to remark that the plates 4 of both rows are 

 placed side by side, are united to plate 3 of row " a " in front, 

 and not at all to plate 3 of row *' ^ ;" but there appears to be a 

 slight junction of the interradial (5) plate '' & " ^ with the large 

 fifth ambulacral of row " a," ambulacrum I. Behind these 

 plates (^ of the odd interradium) are two (5) which contri- 

 bute to the anal opening, and this is completed by the plates 

 6 and 7 of both rows. 



Loven, in dealing with the Spatangoids without an anal 

 fascicle, and which have a peripetalous one only, states that 

 these Prymnadetes present a certain irregularity in the ar- 

 rangement of the plates of the bivium. There is an evident 

 tendency towards a constant disposition in tlie plates called 

 No. 2, in the lateral interradia 1 and 4 ; but this tendency is 

 ehown in a variable and inconstant manner. 



It is the PaJceostoma mirabile, Gray, a form which is ex- 

 ceptional in many respects, that presents the most singular 

 deviation. Whilst all the other Spatangoids that are still 

 existing, excepting Urechinus Naresianus, A. Ag., have in 

 the regular interradia the interradial plate /, at the peristome, 

 followed by a pair of plates, the 2 of row " a " and the 2 of 

 row " Z>," this genus has this second plate of both rows con- 

 founded in one plate^ and this is seen in all the interradia 

 except the odd posterior one (5). Moreover the interradium 

 No. 1, that is the right posterior lateral, has the consolidated 

 plates 2 row " a " and 2 row " h " united to the plate " h " 3. 

 This is the only example of this fusion of three plates that is 

 known, and it does not occur in the other interradia of this 

 species. Inasmuch as this fusion does not take place in the 

 interradium No. 4 of Palceostoma, abnormal heteronomia of 

 the interradium No. 1 occurs. Lovdn terms the union of the 

 plates 2 and 3 of row " a " in the interradium No. 1 a normal 

 heteronomia, and remarks that it occurs in the genera Hemi- 

 aster, Abatus, Agassi'zia, Schizasfer^ and Moira. 



There is a specimen of Palceostoma mirabile in the British 

 Museum, and it most unfortunately is adherent by the actinal 

 surface. The shape of the form and the nature of the apical 

 structures can be seen very well. As in the specimens de- 

 scribed by Lov^n, there are no sutures visible between the 

 apical plates, and the madreporite is not seen between the two 



