Herbert L. Hawkins — Studies on the Echinoidea. 397 



those of Duncan & Sladen are shown to be based upon imperfectly- 

 preserved material. The processes, which may project freely for 

 a varying distance above the rest of the girdle, are slender. The 

 actual ridges are represented, as in Plesiechinus, by the slightly 

 thickened and projecting inner surfaces of the unpaired primordial 

 interambulacral plates. The rest of the interradial portion of the 

 girdle is interpreted as being made of the bevelled edges of the 

 buttresses which radiate over the interambulacra of the adoral 

 surface. These sloping surfaces, here called " false ridges ", are 

 believed to be specialized and somewhat extended as " rests " or 

 " slides " for the inclined pyramids ; thus forecasting the articulation 

 between lantern and girdle found in the Clypeastroida. The 

 retractor muscles were attached to the upper parts of the processes, 

 and it is argued that the protractors must have sprung from the 

 small true ridges, the false ridges having no function as muscle 

 supports. In the absence of positive evidence, radial compass 

 muscles are presumed to have been absent. 



4. List or Papeks consulted. 



Duncan (P. M.). 1884. "On Galerites albogalerus, Lamarck, syn. Echino- 



conus conicus, Breynius" : Geol. Mag., Dec. Ill, Vol. I, pp. 10-18. 



1885. " On the Perignathic Girdle of the Echinoidea" : Journ. Linn. 



Soc. Zool. , vol. xix, pp. 179-212, pis. xxx, xxxi. 



& Sladen (W. P.). 1886. "On the Anatomy of the Perignathic 



Girdle of Discoidea cylindrica " : Journ. Linn. Soc. Zool. , vol. xx, 

 pp. 48-61. 



1889. " A note upon the Anatomy of the Perignathic Girdle of 



Discoidea cylindrica, Lamk., sp. , and of a species of Echinoconus" : 

 Ann. Mag. Nat. Hist., ser. vi, vol. iv, pp. 234-9. 



Hawkins (H. L.). 1909. "On the Jaw-apparatus of Discoidea cylindrica 

 (Lamarck)" : Geol. Mag., Dec. V, Vol. VI, pp. 14S-52, PI. VI. 



1911. "On the Teeth and Buccal Structures in the genus Conulus, 



Leske" : Geol. Mag., Dec. V, Vol. VIII, pp. 70-4, PI. III. 



Loven (S.). 1872. " Etudes sur les Echinoidees " : K. Svensk. Vet.-Akad. 

 Handl., Bd. xi, No. 7. 



1888. " On a Recent Form of the Echinoconidse " : Bih. K. Svensk. 



Vet.-Akad. Handl., Bd. xiii, Afd. iv, No. 10. 



1892. "Echinologica": Bih. K. Svensk. Vet.-Akad. Handl., 



Bd. xviii, Afd. iv, No. 1. 



EXPLANATION OF PLATE XXV. 



Fig. 



1. Copy of the figure of the perignathic girdle of D. cylindricus given by 



Duncan & Sladen (1889). The inner part of the figure, slightly reduced. 



2. Copy of the figure of the perignathic girdle of D. cylindricus given by 



Loven (1892). The inner part of the figure, slightly reduced ; 

 outline only. 



3. The perignathic girdle of D. cylindricus (original). The figure is 



diagrammatic in that the sutures in the ridges were only seen in three 

 of the areas, and the sutures of the coronal plates are omitted. The 

 ridges project very little above the fused buttresses. The processes 

 are intermediate in character between those of the "forma vulgaris " 

 and "forma elatior" of Loven. The specimen is depressed hemi- 

 spherical in shape. The small true ridge is not quite so prominent as 

 represented. 



4. Section through the interradial line of the girdle of a specimen in which 



the ridge is unusually prominent, owing to the feeble development of 

 the buttresses. 



