572 NORTH AMERICAN INDEX FOSSILS. 



having their ambulacral ossicles but sHghtly inclined towards each 

 other and arranged in alternating rows ; the madreporic body is on 

 the oral side. American examples are : Stenaster salteri Billings 

 (Fig. 1910) and S.pulchellus Billings, from the Trenton of Canada, 

 and vS. grandis Meek and Worthen, of the Cincinnatian of Ohio; 

 Palceasterina stellata Billings, of the Trenton; P. rugosa Billings, 

 P. speciosa Miller and Dyer, P. approximata M. and D., all of the 

 Cincinnati group ; Palceaster eucharis Hall, of the Hamilton of New 

 York, P. chemungensis Schuchert, of the Chemung of Pennsyl- 

 vania, and P. crawfordsvillensis Miller, of the Keokuk of Indiana, 

 n. EuASTERi^, with the pairs of ambulacral ossicles opposite 

 each other and inclined at a considerable angle, and the madre- 

 poric body in most cases restricted to the dorsal surface. Amer- 

 ican examples : Pentagonaster browni Weller, from the Fox Hills 

 of Wyoming, and P. f mammillatus Gabb, from the Vincentown 

 beds of New Jersey ; also Asteriasf dubium Whitfield, from Juras- 

 sic sandstones of the Black Hills. 



Branch ECHINOZOA Leuckhart. 

 Class ECHINOIDEA Agassiz. 



The echinoids are free-moving marine echinoderms, with a 

 hollow, globular to disk-shaped shell or test, composed of numerous, 

 thin, closely-joined calcareous plates. They differ in general appear- 

 ance from the crinoids in the absence of stem and arms and in the 

 presence of very numerous superficial spines. 



The main portion of the test is called the corona; it is supple- 

 mented by a system of plates at or near the center of the dorsal 

 or ahactinal surface, and this is known as the apical system (Fig. 

 igii,A,a). The test is pierced by two large openings which in 

 life are covered, except at their centers, by leathery membranes 

 studded with small calcareous particles; these are the mouth open- 

 ing or peristome, and the anal opening or periproct. 



The mouth is on the under or actinal side of the test, either 

 central or excentric in position and is surrounded by a leathery 

 membrane. It contains in life a very complicated dental apparatus 

 {"Aristotle's lantern") consisting of five hard, interradially situ- 

 ated teeth which are in relation with as many pyramids resting 



