478 



NORTH AMERICAN INDEX FOSSILS. 



1. B. carchariaedens Billings. (Fig. 1787.) Ordovicic. 

 Large, attaining a height of 36 mm., and a width of over 40 mm. ; 



section pentagonal ; greatest width at boundary of oral and aboral 



surfaces. (Type of genus.) 

 Chazy of New York and 

 /^ Quebec. 



II. Troostocrinus Shumard. 

 Calyx narrow, elongate, some- 

 what spindle-shaped, with con- 

 tracted subtruncate or slightly 

 convex upper face and with the 

 triangular base flattened on each 

 side. Greatest diameter one 

 third the distance from the sum- 

 mit. Basal plates one third the 

 height of the calyx. Radial plates 

 long and narrow with limbs 

 much shorter than the bodies. 

 Ambulacra short, narrow, deeply 

 impressed. The four anterior 

 deltoids overlapped by the limbs 

 of the radials, the posterior one 

 much larger than the rest and 

 appearing above the radials. 

 Lancet plates entirely concealed by side plates. Spirals five, small, 

 the four anterior more or less divided by the deltoid ridge and the 

 posterior confluent with the anus. Siluric. 



2. T. reinwardti (Troost). (Fig. 1788.) Siluric. 

 Spiracles almost completely divided by the deltoid crests. (Type 



of genus.) 



Niagaran of Tennessee. 



Fig. 1787. Blastoidocrinus carchari- 

 adens, analysis after Hudson. Basal 

 view, showing also star-shaped central 

 summit plate, and the wing plates. 



III. Metablastus Etheridge and Carpenter. 

 Calyx slender, spindle-shaped with greatest circumference nearly 

 half way from the summit. Summit usually acuminate, always 

 contracted; base elongate, triangular, flattened below on all three 



