Perignathic Girale of Discoidea cylindrica. 237 
mentioned. In the British Museum specimen there is (a) no 
median suture in any expansion. ‘There are (8) two sutures 
in the position of those discovered by Lovén. ‘These sutures 
are nearly or quite parallel and include a single median plate. 
The interradio-ambulacral suture arises close to the other 
sutures and bounds the ambulacrum, there being a plate 
between it and the suture @ 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. Lovén. 
Median sutures are so common in the ridge portion (inter- 
radial) of perignathic girdles (see Journ. Linn. Soc., Zool. 
vol. xix. pl. 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. Lovén’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 
