PEEIG]S"ATHIC GIEDLE OF THE ECHTKOIDEA. 195 



arabulacral mediau line. The sutures between the plates to which 

 the pairs belong are seen with benzene, but their ambulacro-inter- 

 radial sutures do not exist ; for the plates, the direction of which is 

 very oblique from above and towards the median line of the am- 

 bulacrum, merge into the mass of the process at their part remote 

 from the median line. Thus in this species, as in the Temno- 

 pleurids, the base of the process is certainly composed of parts of 

 the poriferous zone of ambulacral plates increased in height and 

 crowded. 



It is necessary to admit that the ambulacral plates which are 

 visible at the peristomial part of the process — that is, the first, 

 second, and third plates, and those three others seen on the pro- 

 cess behind it, that is plates 4, 5, 6 of the ambulacrum — may 

 enter into the composition of the base, and of more or less of the 

 upper part of the process in one zone a, in a specimen of nearly 

 adult dimensions. In the opposite process (fig. 17) two plates 

 are in front and three behind the process, and none have ambu- 

 lacro-interradial sutures ; they compose the process of zone 6. 



The perignathic ridges of this species are low, and they are 

 curved downwards at the upper free and narrow edges. The width 

 of a ridge at its edge from suture to suture is less than the width 

 of the peristomial part of the corresponding interradium. On the 

 peristomial face of a ridge there is a swelling at and on either 

 side of the median line and just above the actinal edge. On 

 either side of this there is a groove which is continuous with a 

 branchial cut, and above the swollen part there is a concavity sur- 

 mounted by long markings for the attachment of the protractor 

 muscles (fig. 16). 



On examining the other side of a ridge, or from the circumfer- 

 ence inwards, it is to be noticed that there is no median suture 

 near the edge, and that under benzene certain plates become very 

 distinct. A single plate, which is relatively much less developed 

 in height than in the Temnopleuridse, forms the whole edge of a 

 ridge, and it varies in height according to age. Eollowing this 

 plate, in one zone (a), there is a low plate and a higher one, and all 

 the low plate and a small part of the next enter into the formation 

 of the ridge's base. On the other zone a large plate succeeds the 

 single one, and part of it rises in the base. (Figs. 24 and 18 : the 

 transverse line a is the level of the rise of the base. Provisionally 

 the plates of fig. 24 are numbered as if they were in direct and 

 normal succession ; the single plate is 1, and it is followed by 2, 



16* 



