Perignathic Oirale of Discoidea cylmdrica. 237 



mentioned. In the British Museum specimen there is (a) no 

 median suture in any expansion. There are (/Q) two sutures 

 in the position of those discovered by Loven. These sutures 

 are nearly or quite parallel and include a single median plate. 



The intervadio-ambulacral suture arises close to the other 

 sutures and bounds the ambulacrum, there being a plate 

 between it and the suture (8 triangular in shape, with its 

 point at the branchial incision. The dimensions of these 

 triangular plates varies, and in no instance is there the small 

 additional plate described by Prof. Loven. 



Median sutures are so common in the ridge portion (inter- 

 radial) of perignathic girdles (see Journ. Linn. Soc, Zool. 

 vol. xix. pi. xxxi.) that their presence would have been an- 

 ticipated in Discoidea ; but the variability of the girdle seems 

 to extend to the suturing. 



According to Prof. Loven's view the plates on either side 

 of the median suture of the interradial expansion of the peri- 

 gnathic girdle are truly interradial, and the plates on either 

 side of them are ambulacral. 



According to the terminology suggested and employed in 

 the researches on the nature of the perignathic girdle of the 

 Echinoidea (Journ. Linn. Soc, Zool. vol xix.) the plates on 



