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



The perignathic girdle is better preserved in some parts than 

 in the specimen originally described by us ; but while the 

 pores of the ambulacra are not so clearly placed as in the 

 early specimen, there are, beyond a doubt, sutures in the inter- 

 radial expansions (the ridges). 



1. Taking the old and this new specimen as examples, it 

 is shown in them that the interradio-ambulacral suture is 

 distinct and that the pairs of pores are between it and the 

 median suture of the ambulacrum. 



It appears, then, that no part of the expansions is truly 

 ambulacral ; all is interradial. 



Very respectfully we would draw our friend's attention to 

 his drawing, fig. 2. There is a sliglitly oblique and not quite 

 transverse suture separating the ambulacral plate of zone b 

 nearest the peristome from the expansion. In our opinion 

 that suture is the natural limit of the ambulacral region and 

 is interradio-ambulacral. Consequently the plate which this 

 suture- bounds actinally is interradial and not ambulacral. 



2. As the evidence of the facts just noticed is clear and 

 the drawing given by Prof. Loven is doubtless correct, we 

 must admit that variation is possible in the construction of 

 the girdles of these forms, which are considered to be Gnatho- 

 stomes by some and to be Edentates by other naturalists. 



3. In the specimen originally examined by us there is not 

 the slightest vestige of sutures in any one of the expansions, 

 and we had no right to assume that there were any. 



Prof. Lov^n shows {op. cit. p. 9, fig. 1) that there are 

 certainly three sutures in each expansion, irrespectively of 

 what may be considered as interradio-ambulacral sutures. 

 Possibly there are two others uniting small triangular pieces 

 to the outer edges of expansions. 



In the lately cut specimen at the British Museum there are 

 distinct sutures in the interradial expansions, but their distri- 

 bution differs from that figured by Prof. Lovdn. 



Prof. Lov€n shows that in each expansion there is (a) a 

 median suture, (/3) a suture on either side of the median suture, 

 being parallel at some distance. Each of these two sutures, 

 which are parallel with the median one, bounds a plate at the 

 side of the median suture which unites the plates. Each one 

 of these two sutures seems to start from close to the inter- 

 radio-ambulacral suture at each branchial incision, a small 

 space intervening. 



According to Prof. Loven's drawing (fig. 1) the interradio- 

 ambulacral sutures are curved, and they limit a plate, a con- 

 siderable part of which is free towards the ambulacrum and 

 which has upon its corner the little plate and suture already 



