474 NORTH AMERICAN INDEX FOSSILS. 



terradial). One small plate is interpolated between the second 

 and third cycles below the interradial. Arms five, feeble, uni- 

 serial, rising from the five radials. Pore-rhombs about 22, at 

 the intersection of three (not two as usual in cystoids) plates, 

 in the form of equilateral spherical triangles ; their diameter about 

 2 mm., the ducts passing obliquely across the margin of the plates. 

 Ordovicic. (Sometimes classed with crinoids.) 



35. P. conicus Billings. Ordovicic. 

 Length of calyx 11 mm.; diameter at base 3 mm.; diameter at 



top 8 mm. (Type of genus.) 

 Trenton of Ottawa. 



XIX. ZoPHocRiNUS S. A. Miller. 



Calyx ovate or pear-shaped, consisting of two circles of plates, 



the three basals and four plates of the second circle, and the 



ventral disk. The distal margins of each 



plate of the second circle are pierced by 



[ZID^^^CZI! five pores. Ventral disk consists of a circle 



of 20 minute plates through each of which 



a pore passes perpendicularly, connecting 



Fig. 1785. Zophocrinus ^j^^ the pores that pierce the beveled edges 



r Aft^r Well x^ °^ ^^ plates below. Within this circle of 



20 plates are other smaller plates which 



comprise the central portion of the tegmen. The pores piercing 



the plates seem to ally the genus to the cystoids more closely than 



to the crinoids. Siluric. 



36. Z. howardi S. A. Miller. (Fig. 1785.) Siluric. 

 Small, less than one half inch long. (Type of genus.) 

 Niagaran of Indiana and Illinois. 



Class BLASTOIDEA Say. 



The Blastoids are an entirely extinct group of marine echino- 

 derms. They are confined to Palaeozoic time, being found from 

 the Ordovicic to the Carbonic, but reaching their climax in the 

 Mississippic. 



They had an ovate or bud-like body {^calyx'), no arms, and were 

 short-stalked or stemless. They differ in general appearance from 

 the Crinoids in the total absence of arms. 







D 



