PEEIGNATHIC GIRDLE OF THE ECHINOIDEA, 207 



The peristomial face of the ridges (for such they are) is usually 

 raarked by a depression on either side of their median line, and 

 the other face is convex. The tops may be rounded, pointed, 

 or rectangular, and the breadth of a process is greater than 

 the measurement from the peristomial face backwards. 



There are no traces of small plates in the ridges. 



Probably the depressions just mentioned are the points of 

 attachment of muscular fibres, and it is evident that the motions 

 of the jaws must be as restricted in this species as in the true 

 Clypeasters. 



On looking at the Tphotographs of ^cMnaracJinius parma, E. mi- 

 rahilis, Jifellita testudinata, and both Ecliinodiscus aurihcs and E. 

 hiforis in the ' Eevision of the Echini,' by A. Agassiz, it is perfectly 

 evident, after the study of the JLaganum, that they all have the 

 projections single and one in each interradium, and that they 

 are all homologues of the perignathie ridges of the regular 

 Echinoidea. 



VII. Conclusions, 



As the structures which give attachment to the muscles which 

 protrude and retract the jaws of the Echinoidea, and which 

 are integral and not additional parts of the test surrounding 

 the peristome within, are not homologous in Cidaridse, Echi- 

 nidse, Clypeastridse, and Laganidse, it is impossible to retain 

 the old term of " auricles." As the structures form a perfect 

 girdle around the jaws in Discoidea, and more or less dis- 

 connected parts of a girdle in other forms, the term " perignathie 

 girdle " is advisable. The girdle consists of " processes " usually 

 united above, but sometimes, and mostly in young forms, dis- 

 connected, and of " ridges " which connect the " processes " on 

 the side remote from the ambulacra. 



In the Cidaridse the muscular attachments are all on perignathie 

 and usually disconnected "ridges," which are modifications of the 

 peristomial interradial plates. The ridge is made up of two plates 

 in one interradial zone (plates 1 and 2), and of one plate (1) in the 

 other, there being a median line. The plates of the ridge are 

 the upward growths of the plates just numbered, and which carry 

 tubercles at the peristomial margin actinally. The " ridges " may 

 overhang so much as to join and arch over the ambulacra, as in 

 the specimen of Phyllacanthus, 

 ■ The Cidaridse have no " processes." 



