230 Prof, P. M. Duncan and Mr. W. P. Sladen on the 



With regard to the critical plates of interradiiim No. 1, it 

 may be observed, in Lov^n's exquisite drawings (pi. xvi. 

 figs. 185 and 187, ' On Pourtalesia &c.'), that the broad first 

 plate of interradium No. 1 enlarges in width slightly, and 

 then contracts, being nipped in between the ambulacral plates 

 on either side ; it is narrower at that suture which is remote 

 from the mouth than at the peristomial margin. 



With regard to the ambulacra of Paheostoma. On exami- 

 ning Loven's plate (' Etudes,^ xxxii. fig. 197) of the spread- 

 out diagram of the test of Palceostoma mirahile^ and studying 

 the ambulacra I. and V., one is struck with the difference 

 between them ; the ambulacrum V. presents nothing unusual 

 except that the plates 2^ -3, 4 of both rows^ "a" and "6," 

 are broader than is usual — in Hemiasler^ for instance. The 

 plates are large in relation to those of the corresponding inter- 

 radium, and the No. U of row " h " tits in with tlie posterior 

 lateral suture of the large sternal plate No. 2 of the odd inter- 

 radium, row " a," and the edge of the plate No. S. With 

 regard to the ambulacrum No. I., the plate 4 of row " a " is 

 moderate in size, and comes in contact with the antero-lateral 

 suture of tiie interradial (5) plate No. 3, row " Z;." 



The next ambulacral plate, No. o, row " «," is very large^ 

 much larger than the corresponding plate of the row " h " of 

 ambulacrum V. ; it is broad, and indeed broader than long, 

 and it projects towards the median line. It occupies a part 

 of the space which is left, in consequence of the peculiar 

 arrangement of the plates of the odd interradium, at the pos- 

 terior part of the sternum, so that its inner part appears to 

 form a portion of the odd interradium (5). The postero- 

 lateral suture of this large ambulacral plate touches the side 

 of plate 4 of interradium 5, and much of it fills up the space 

 between plates 3 and 4 " 5 " of interradium 5. 



On looking at Loven's drawing of the peristomial surface 

 of Palceostoma ('CEfversigt af K. Vetensk.-Akad. Forhand- 

 lingar,' 1867, p. 454) the remarkable breadth of the ambu- 

 lacra Nos. I. and V. contrasts with the breadth of the plates 

 of the intcrradia 1 and 4 &c. An ambulacral plate is as 

 broad as the first or peristomial plate of any interradium. 



In plate xxxii. of the ' Etudes,' tig. 197, the following details 

 can be observed : — The odd or posterior interradium (No. 5 of 

 Loven) is exceptionally formed, and it does not constitute 

 a plastron ; moreover, the arrangement of the plates does not 

 resemble that of other Spatangoids. The plate 1 is broad, 

 but not broader at the peristomial margin than the other 

 first plates of the interradia, but it is long. Behind are 

 plates 2, but that of row " a " is longer than that of row " i," 



