MUSEUM OF COMPARATIVE ZOOLOGY. 41 



Fig. 20. Hyolithes Billingsi, Walcott ? three specimens with cross-sections. 



Fig. 21. Ptychoparia mua-onatus, sp. n. ; a, glabella, with very marked terminal 

 spine, smaller type ; b, glabella, larger type ; c, movable cheek associated with the 

 same ; d, specimen preserving thoracic segments. 



Fig. 22. Salterella curvatus, sp. n. ; two specimens, the lower slightly coiled. 



Fig. 23. Hyolithellus micans, Billings. 



Fig. 24. Not numbered in the plate, a movable cheek, relationship unknown. 



Fig. 25. Hyolithes princeps, Billings ; a, ventral view ; b, sections showing in- 

 vagination of different individual shells of same species ; some of the smaller with 

 more circular outlines may also be those of Hyolithellus micans, Billings. 



Fig. 26. Microdiscus speciosus, Ford ; head ; figure introduced as a means of com- 

 parison with 21. belli-marginatus. From Olenellu? Cambrian of Troy, N. Y. 



