58 PItOF. P. M. DUNCAN AND Mil. W. P. SLADEN OJf THE 
true that the ambulacral parts of the girdle are perforated by one 
or more pairs of tentacular pores ; and therefore the parts thus 
penetrated are the homologues of the processes which in other 
Gfnathostomes (omitting the Cidaridse) form the sides to or 
processes of the arches (“ auricles ” of authors). 
The interradial portions of the girdle in Discoidea cylindrica 
are the homologues of the ridges of the other regular Echinoidea, 
including the Cidaridse, and, as in the other forms, the ridges 
are composed of one or more interradial plates. 
On comparing the girdles of Cidaris and Discoidea, it will 
he noticed that both have the ridges well develojted, and clearly 
in consequence of the attachment of protractor muscles. In 
Cidaridse the utility of the retractor muscles is diminished by 
the nature of the scaly structure around the peristomial opening ; 
and in Discoidea the extremely high and narrow peristomial tube 
and the spines attached to tubercles within indicate that wide 
opening of the jaw-ends could not occur even as much as in 
Cidaris. 
Neither in Cidaris nor in Discoidea are the “ processes ” of the 
ambulacral parts of the girdle developed so as to afford origin or 
attachment to muscles ; and it is evident that the portions of the 
ambulacra in Discoidea which are perforated, and which are the 
homologues of the processes of the ambulacral arches of the 
Echinidae, could not have given attachment to muscles because of 
the presence of the pores ; moreover, the processes of Echinidse 
are not the origins of muscles, but the arches are. The distinction 
in structure between the girdles of Discoidea and of the Echinida?, 
such as the species of the genera Echinus, Salmacis, Temnojpleurus, 
Diadema , Ccelopleurus, Stronyylocentrotus, &c., is evident ; for 
there are no arches and no upward prolongations or jjrocesses 
to form arches in Discoidea. The greatest affinity in structure 
is with the Cidaridse ; but the difference is important, inasmuch 
as the ambulacral parts of the girdle are wanting in Cidarids, 
and are high and block the ambulacral path in Discoidea. 
No trace of pyramids or of any portions of a dental apparatus 
has ever been found in Discoidea ; but it is impossible to reject 
the supposition that the genus was gnathostomous, for the pre- 
sence of ridges marked with depressions in the girdle would seem 
to point to muscular attachments and to the inevitable presence 
of jaws. The position of the jaws was probably not so vertical as 
in the Cidaridse, but slanting more or less ; and it is probabJe 
