Herbert L. Hawkins — Studies on the Echinoidea. 395 



providing a smooth, slippery surface on which the splayed pyramids 

 of the lantern could recline. 



(d) The position of the jaio -muscles. (PI. XXV, Fig. 6.) 

 There is a clearly defined articular facet on the upper part of each 

 process which undoubtedly served for the attachment of the retractor 

 muscles. I have failed to find any other indications of muscle- 

 attachments upon the processes ; and, to judge from the known 

 characters of the pyramids, it seems improbable that a second pair of 

 retractors was present. 



The position of attachment of the protractor muscles is less clear. 

 In those Clypeastroida which have paired processes, the protractors 

 arise from the interradial sides of the processes, usually at or near 

 their bases. Such a position for the protractors is quite impossible 

 in Discoides, since there are no "interradiad faces" to the processes 

 owing to the great height of the "false ridges". Hence the 

 protractors must have been attached to some part of the ridge or 

 " false ridge " ; that is, in the Diademoid manner. I have not been 

 able to recognize any muscle-impressions on the interradial parts of 

 the girdle. Clearly the broad, shallow, elliptical depression which 

 occupies most of the surface of the false ridge is not a muscle-scar. 

 In some specimens there are very small and inconspicuous knobs on 

 the false ridge at the upper corners of this central depression. They 

 do not, however, suggest muscle-attachments, since these are 

 normally roughened and depressed in Echinoid girdles. There 

 remains the somewhat prominent " true ridge ", on the actual 

 margin of the peristome. If the interpretation of the girdle given 

 above is correct, this is the proper situation for protractor attach- 

 ments, by analogy with the Diademoida. It will be seen that in 

 PI. XXV, Fig. 6, I have based these muscles upon this median 

 thickened plate. A further reason for this reconstruction — one that 

 seems to me to be very cogent — is the normal direction of the 

 protractor muscles. These, in the Diademoida, pass almost vertically 

 from the low ridges to the tops of the hemi-pyramids. Owing to 

 the pi'ominence of the true ridge in Discoides, such a muscle arising 

 from it would have free play for a considerable distance in all 

 directions (see PI. XXV, Fig. 6). Owing to the concave curvature of 

 the rest of the "ridge ", the only other position from which similar 

 freedom could be attained would be the actual crest — a most unlikely 

 place for the attachment of a strong muscle. 



There is no evidence for the existence of radial compass muscles, 

 nor of compasses in the lantern. Although negative evidence, when 

 concerned with structures of such delicacj', is utterly unreliable, 

 I have omitted them from the restored figui'e in view of their 

 proved absence in the Clypeastroida. If it should be shown that 

 Discoides had compasses, I should incline to find the attachment 

 of their muscles on the true ridges between the pairs of protractors. 



(e) The nature of the "false ridges" . 

 If the foregoing argument respecting the position of the muscle- 

 attachments of the perignathic girdle of Discoides is accepted, the 

 interradial portion that I have called the false ridge is seen to be 



