578 NORTH AMERICAN INDEX FOSSILS. 



ttf. Periproct supramarginal, oval, anterior ambulacrum in groove 



JttJ. Anterior groove shallow, periproct not sunken. 



XXVII. Holaster. 

 XXXX- Anterior groove deep, with angular margin ; periproct in 



depression in truncated posterior face XXVIII. Cardiaster. 



7*. Ambulacra petaloid or subpetaloid, divided into trivium and bivium 



anterior petal differing from others in form and characters 4t. 



4t. Without fascioles 5| 



5j. Periproct supramarginal ; ambulacra petaloid dorsally, with 



elongate unequal pores XXIX. Epiaster, 



5j. Periproct in posterior truncated area; posterior lateral ambu- 

 lacra short XXX. Enallaster, 



4t. With fascioles, except in subanal area 6J 



6J. Anterior ambulacrum in shallow groove ; pores oblique. 



XXXI. Hemiaster. 

 6J. Anterior ambulacrum in deep groove ; pores round and small 



XXXII. Linthia 



4f. With subanal fasciole 7j. 



7j. Ambulacral petal more or less triangular, anterior ambu- 

 lacrum in groove XXXIV. Echinocardium. 



7j. Ambulacral petals not triangular, anterior ambulacrum nearly 

 obsolete, and nearly flush XXXIII. Maretia, 



Subclass Palaeechinoidea Zittel. 

 Order PERISCHOECHINOIDA McCoy. 

 I. EociDARis Desor. 

 Ambulacra with two columns of plates, with pores near outer 

 extremity; interambulacral plates in four or more columns, pent- 

 agonal at margin, otherwise hexagonal, with large tubercle on each 

 plate, smooth at base, perforated at summit; distinguished from 

 ArchcBOcidaris by absence of a definite areole and ring; spines 

 slender, ornamented with small, sporadic spinules. Devonic-Permic. 

 I. E. halliana Geinitz. Carbonic. 



Minute interambulacral plates with median primary spine-boss 

 surrounded by an open ring of secondary smaller ones; primary 

 spines slender, surface finely striate, the striae spinulose; length 

 of spines 8.5 mm., diameter of plate 2.5 mm. 



Upper Coal Measures of Nebraska, Missouri, Iowa and Colo- 

 rado. 



11. Arch^ocidaris McCoy. 

 Test large, regular and globular ; ambulacra .reaching the mouth, 

 straight, with two columns of pore-bearing plates, each with a pair 



