58 PROF. p. M. DUNCAN AND MR. W. P. SLADEN ON THE 



true that the ambulacral parts of the girdle are perforated bj one 

 or more pairs of tentacular pores ; and therefore the parts thus 

 penetrated are the homologues of the processes which in other 

 Grnathostomes (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 

 be noticed that both have the ridges well developed, 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 Echinidae, 

 such as the species of the genera JEcTiinus, Salmacis, Temnopleurus, 

 Diadema, Ccelopleurus, Strongylocentrotus, &c., is evident; for 

 there are no arches and no upward prolongations or processes 

 .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 



