464 



MR. HERBERT L. HAWKINS ON 



the later genera, it is impossible to regard the adorally situated 

 periproct of E. d&pressus as a case of overspecialization. Most 

 of the species of Gcenholectijpus have the periproct in that 

 position, as have all the forms of Discoidea. The H. hemi- 

 sphcericus character reappears in Conidus, and is retained in most 

 of the Cretaceous species of Pyrina. 



Text -fie. 56. 



Diagram showing the shape and position of the periproct in A. Tygaster sens, sti'., 

 B. Megapygus or Macropygus ; C. Anorthopijgus ; D. Holectypus {liemi- 

 sphcericus) ; E. Conulus ; P. Holectypus (depressits) ; G. Discoidea. The 

 thick line represents the ambitus. The proportionate height of the inter- 

 radial plates is indicated. 



Loven (36) has indicated the correspondence in periproct- 

 migration that exists between the Holectypoida (his Echinoconidse) 

 ancl both the Echinoneidse and Cassidulidfe. Galeropygus and 

 Clypeus both have the Pygaste7'-\ike periproct, in contact with 

 the apical system, while the Nucleolitidse show an arrangemetit 

 more like that of Pileus. The Echinolampidje include forms in 

 which the periproct may be marginal or adoral in position. 



In connection with the position of the periproct, an interesting 

 feature of the posterior interradius may be discussed. Reference 



