12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



ally, with about 15 covering plates to a row, all arched along the 

 median line of junction. The species is so much like H. rectiradiatus 

 (Shideler) (pi. 5, fig. 10) from the Richmond group that differences 

 are hard to note. When magnified to the same diameter and com- 

 pared, H. rectiradiatus seems to have slightly longer, narrower, less 

 tapering ambulacra. Related also to H. youngi, it dififers in having 

 longer, more slender rays. 



Occurrence. — Trenton (upper part Prasopora zone), near Jackson 

 Park, Peterborough, Ontario. 



Plastotypes. — U.S.N.M. no. S-3892. Originals in Victoria 

 Memorial Museum, Ottawa. 



HEMICYSTITES (LEBETODISCUS) YOUNGI (Raymond), 1915 

 Plate 3, fig. I 



Although probably related to H. chapmani (pi. 3, fig. 9), the rela- 

 tively shorter and broader rays will distinguish H. youngi. Compared 

 with H. hillingsi, a very similar species, H. youngi differs in the 

 broader rays and in lacking the large punctate marginal plates. The 

 short, wide ambulacra and the unusual size and length of the ambu- 

 lacral covering plates, as well as the overlapping interambulacrals, 

 readily characterize this species. 



Occurrence. — Middle Trenton, Eldon Township, Victoria County, 

 Ontario. 



Plastotype. — U.S.N.M. no. S-3896. Original in Victoria Memorial 

 Museum, Ottawa. 



HEMICYSTITES (AGELACRINUS) BILLINGSI (Chapman), i860 

 Plate 3, fig. 2 



The photograph on plate 3 of Raymond's plesiotype of this species 

 indicates a Hemicystitcs similar to H. (Lebetodiscus) youngi (pi. 3, 

 fig. i) but distinguished by its larger marginal and interambulacral 

 plates, its shorter rays, and by the pitted condition of all the plates. 



Occurrence. — Trenton (Cystid beds), near Jackson Park, Peter- 

 borough, Ontario. 



Plastotype. — U.S.N.M. no. S-3891. Original in Victoria Memorial 

 Museum, Ottawa. 



HEMICYSTITES CURTUS, n. sp. 

 Plate 4, fig. 8 

 This minute species, which occurs as a slightly convex theca about 

 6 mm in diameter attached to a limestone slab, may be distinguished 



