MUSEUM OF COMPARATIVE ZOOLOGY. Al 
Fig. 20. Hypolithes Billingsi, Walcott? three specimens with cross-sections. 
Fig. 21. Ptychoparia mucronatus, sp. n.; a, glabella, with very marked terminal 
spine, smaller type; 4, 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 JZ. belli-marginatus. From Olenellus Cambrian of Troy, N. Y. 
