488 NORTH AMERICAN INDEX FOSSILS. 



XII. Orophocrinus von Seebach. 

 Like Codas ter in general shape and arrangement of plates, but 

 basal plates forming a lower cup and spiracles only ten and linear, 

 one on each side of an ambulacrum. Upper parts of lancet plates 

 exposed but often closed distally by the numerous side plates. 

 Stem round, composed of short and rounded or slightly pentagonal 

 joints. Mississippic. 



28. 0. stelliformis (Owen and Shumard). (Figs. 1801, 1802.) 



Mississippic. 



Calyx balloon-shaped. Base narrow, expanding gradually to 

 the basiradial sutures and thence rapidly to the radial lips. (Type 

 of genus.) 



Burlington of Missouri and Iowa. 



29. 0. campanulatus (Hambach). Mississippic. 

 Differs from 0. stelliformis in the bell-shaped calyx, shorter 



ambulacra and less prominent radial lips. 

 Burlington of Missouri and Iowa. 



Class CRINOIDEA Miller. 



The crinoids or sea lilies are marine invertebrates, represented 

 in the modern seas by a number of genera and species which range 

 from shallow water to a depth of about 3,000 fathoms. They 

 live in colonies and are usually of very local distribution. They 

 are usually attached by a long stem, rarely by the cup directly, or 

 free-swimming. 



The skeleton or test of a crinoid consists of the stem or column 

 and the crozvn. When lateral appendages are present on the stem 

 they are called cirri; those of the lower or distal end are radicular 

 cirri and form the root. The stem is composed of joints which 

 may be of uniform or varying size. In modern species nodes and 

 internodes may generally be recognized, the former bearing the 

 cirri, and the latter appearing between the nodal joints. In forrn 

 the plates often change with growth, the primitive form being 

 shown by the joints beneath the calyx. The axial canal passes 

 through the center of the stem, and is variously shaped. 



The crown consists of calyx and arms; the former encloses the 

 visceral cavity, the latter constitute the free appendages radiating 

 from the calyx. 



