348 Herbert L. Hawkins — Studies on the Echinoidea. 



faces. Retractors based upon the two sides of an arched " auricle " 

 serve hemi-pyramids belonging to two separate maxilla?, and so 

 normally diverge. The planes of the interradial faces of the 

 processes in Plesiechinus are themselves directed away from one 

 another, so that a divergent pair of retractors could spring from them 

 practically at right angles to the articulating surface. 



The protractor muscles must have been based either upon the 

 incipient ridges (as they would be in Diademoida), or else far back 

 and low down on the interradial sides of the processes (after the 

 manner of Clypeastroida). The feebleness of the interradial elements 

 of the girdle in Plesiechinus would tend to suggest the latter 

 alternative, but nevertheless I incline to believe that they had the 

 former position. In Discotdes and Conulus there is no "interradial 

 face" to the process, which rests against, and is sutured to, a ridge- 

 like thickening of the test of practically the same height as itself. 

 And the two concave bays, resembling " combes" on an escarpment, 

 that occur on the adoral surfaces of the small ridges, seem to demand 

 an explanation which is most satisfactorily given by calling them 

 muscle-impressions. Plesiechinus is far more like a Diademoid than 

 a Clypeastroid in general characters, so that the balance of probability 

 would place the protractors on the ridges, insignificant though 

 they are. 



The only other series of jaw-muscles that are attached to the 

 perignathic girdle in Diademoida are the slender "radial compass 

 muscles", which diverge from the forked ends of the compasses. 

 They spring from the ridges, above and behind the protractors, in 

 the Hegularia Ectobranchiata. The two slight depressions on each 

 side of the central interradial knob are extremely suggestive of sites 

 for the attachment of these muscles. But if the depressions had this 

 purpose their presence would imply the existence of compasses in the 

 lantern of Plesiechinus. Compasses are not known in Discoides 

 (though their fragile nature might well account for their non- 

 preservation), and they are definitely absent from the Clypeastroid 

 lantern. The existence of radial compass muscles in Plesiechinus 

 must therefore be regarded as problematical. In the accompanying 

 restoration, I have inserted them as dotted lines in the position 

 that they would presumably have occupied if they were developed. 



There is no indication of the existence of any additional retractor 

 muscles, such as occur in the Clypeastroida, but on the other hand 

 there is no reliable proof of their absence. The processes, apart from 

 their supporting buttresses, are so slender that they seem unlikely 

 to have given support to more than the single retractors. 



3. The Perignathic Girdle of Pygaster and Holectypus. 



As far as can be judged, there is no essential difference in the 

 position and relative proportions of the elements of the girdle in 

 Pygaster semisulcatus from that already described. I have studied 

 the apparatus only from internal moulds of this species. 



In Holectypus hemisphcericus and depressus I have succeeded in 

 clearing the inner parts of the peristome sufficiently to reach the 

 adoral edges of the processes, which are like those of Plesiechinus in 



