PLATE IV. 



Fig. 



la. Perisphinctes Davidsoni, S. S. Buckman, showing the 

 mouth-border and the depression where the test is 

 removed. Zone of Steplian. Humphriesianum. Oborne, 

 Dorset. My collection, one-half natural size. There are 

 indications of ribs all over the specimen. 



Ib. The same, showing ventral area. 



20. Perisphinctes Davidsoni, young forms, showing the mouth 



** 



border. Zone of StepTtani. HumpJiriesianum. Oborne, my 

 collection. 



2b. The same, showing ventral area. "Where the test is 

 removed the ribs do not seem to meet across. When it is 

 on, however, they do. Natural size. 



