462 



NORTH AMERICAN INDEX FOSSILS. 



5. G. glans Hall. (Fig. 1768.) 



Elongate, abruptly expanding at summit, 

 to three inches. (Type of genus.) 



Niagaran of Wisconsin. 



Siluric. 

 Length of calyx one 



IV. Paleocystites Billings. 

 Calyx oval or pear-shaped. Plates numerous and furnished 

 with pore-rhombs ; the pores penetrate the margins of the plates 



-Fig. 1768. Gomphocystites glans, in- 

 ternal mold, X Vz- (After Hall, 20th 

 Museum Report.) 



Fig. 1769. Ariiygdalocystites florealis ; 

 m, mouth, left and posterior views. (Af- 

 ter Billings. ) 



and extend to the center but do not open on the exterior surface ; 



this gives the edges of the plates when viewed from within, a 



notched appearance. The ducts pass vertically across the sutures. 



Ordovicic. 



6. P. tenuiradiatus (Hall). Ordovicic. 



Calyx pear-shaped, with upper part largest. Length about two 

 inches. Plates somewhat hexagonal, depressed conical ; when 

 slightly worn they are covered with deep striae — the cut edges of 

 the pore ducts. (Type of genus.) 



Chazy of New York and Quebec. 



V. Amygdalocystites Billings. 

 Calyx flattened laterally. Plates without pores, numerous, and 

 irregularly arranged. Ambulacra large, recumbent, composed of 

 a large and a small series of plates, the small being situated on the 

 edge and below the large series. Ambulacral opening at apex, 

 rear the mouth. Ambulacral groove not in the middle of the 



