476 NORTH AMERICAN INDEX FOSSILS. 



between the two posterior spiracles (Fig. 1786,0). In other types 

 {Pentremites, Granatocrinus) the anal opening is shared by the 

 then enlarged posterior spiracle (Fig. 1799,0). 



Each ainbidacruin consists of a medially placed harrow plate, 

 {lancet plate) running its entire length ; upon the outer edges of 

 this plate rests a row of small elongated side plates and upon these 

 in some genera a series of still smaller plates, the outer side plates. 

 The lancet plate is often concealed by the side plates (Fig. 1786, h) 

 so that nothing or only a small part of it along the longitudinal 

 groove (food-groove) is visible. The sutures between the side 

 plates are indicated by shallow grooves extending from each side 

 of the ambulacrum to the median groove. Small pits or tubercles 

 present on the side plates, indicate the places where the small 

 jointed appendages (pinnules) were formerly attached; when, 

 as rarely happens, they are fully preserved, they entirely conceal 

 the ambulacral surface. 



Piercing the outer edge of the side plates, or the outer side 

 plates when these are present, are marginal pores (Fig. 1786, b) 

 which enter a suspended longitudinal tube or bundle of parallel 

 tubes beneath. These tubes {hydr aspires) begin at the lower 

 end of the ambulacrum, run parallel with its sides, and terminate 

 in the two spiracles bordering its upper end ; the function of these 

 was probably respiratory. 



The stem is very rarely preserved ; it is round, provided with 

 a small central canal, and composed of short joints (Fig. 1793,0). 



Literature. 

 1886. Etheridge, R., and Carpenter, P. H. Catalogue of the 



Blastoidea, London, pub. by British Museum. 

 1903. Hambach, G. Revision of the Blastoideae, with a Proposed 



New Classification, etc. Trans. Acad. Sci. of St. Louis, Vol. XIII., 



pp. 1-68, pis. I.-VI. 



See also papers by Barris (Davenport Acad. Sci. Proc.) and the 

 various State Reports. 



Artificial Key to Genera. 



A. Base tapering ; calyx spindle-shaped or club-shaped i. 



I. Greatest diameter of calyx nearer summit than base a. 



a. Summit broad *. 



*. Five round spiracles present V. Pentremitidea. 



*. No round spiracles present f. 



f. Base forming a low cup „ XII, Orophocrinus. 



