Herbert L. Haivkins — Studies on the Echinoidea. 439 



boundary suture between the platform and the normal inter- 

 ambulacrum, and (3) an interradial suture bisecting the carina. In 

 addition, there appears to be a suture between the prominence and 

 the carina, but it is by no means clear, although the junction between 

 these two parts of the structure is very sharply defined. 



The only additional ingredient of the girdle is the narrow, but 

 proximally striking, carina which occupies the line of the perradial 

 suture. The bulbous proximal end of this carina extends inwards 

 beyond the normal circumference of the peristome. It is presumably 

 developed for the stiffening of the thin coronal plates of the 

 ambulacrum, which would otherwise seriously weaken the cohesion 

 of the adoral parts of the test ; but it may possess a further function, 

 to which reference will be made later. The carina covers and quite 

 obliterates the perradial suture. 



(c) The Homology and Function of the Structures. 



A comparison of the elements of the girdle of Conulus with those 

 of Discoides (see Part V of this series) reveals an essential similarity. 

 The processes are quite obvious. The central interradial prominence 

 on the peristome-border is clearly the "true ridge", while the 

 remainder of the interradial structure represents the " false ridge ". 

 Assuming the existence of a lantern, the retractors will have been 

 attached to the upper parts of the processes, and the protractors to 

 the true ridges. The false ridges, by analogy with those of JDiscoides, 

 will have served as a rest or slide for the inclined pyramids. The 

 only serious difference in the girdle-structure between Discoides and 

 Conulus is found in the false ridges. In Discoides each false ridge 

 is practically an inclined plane, with a very faint median concavity. 

 In Conulus it has a concave surface, with a median carina, deeply 

 excavate sides, and an overhanging summit. I believe that these 

 three features are all to be associated with the curious nature of the 

 buccal plates. The pyramids of the lantern would rest upon the 

 true ridges and the gently inclined summits of the false ridges, and 

 so would leave considerable spaces between themselves and the deep 

 hollows, and even the carinas, of the slopes of the false ridges. 

 When the mouth was opened the massive buccal plates would 

 inevitably need to be shifted, and it is hard to imagine more likely 

 or suitable places for them than the deep slots in the false ridges. 

 The median carina and the projecting edges of the processes would 

 ensure their sliding in the right direction (so as to avoid confusion 

 with the protractor muscles), and the proximal bulb of the perradial 

 carina might have served to start them on their proper course when 

 retracted. Thus the view put forward by me in 1911 as to the 

 movement and disposal of the buccal plates still holds good, although 

 I now disbelieve in the homology between those plates and the 

 lantern-pyramids. 



If the lantern was of a " flaring" nature (as would be probable in 

 view of its character in Discoides and the Clypeastroida), it must 

 perforce have come very near to, if not into contact with, the upper 

 part of the perignathic girdle. "Whether it articulated with the 

 glossy knobs at the summits of the processes is a matter of doubt, 



