REVISION OF PALEOZOIC STELLEBOIDEA, 



267 



Formation and locality. Okaw Bluffs, between Chester and 

 Kaskaskia, Illinois, in the Chester formation of the Upper Missis- 

 sippic. The holotype is in the Illinois State collection, No. 2480. 



Subclass OPHIUROIDEA. 



These animals are not present in the older Paleozoic, may have 

 appeared in the late Devonic, and <Jo not seem to have been abundant 



FIG. 33. OPHIOTERESIS. AFTER BELL FROM GREGORY. 

 ABORAL SURFACE OF AN ARM OSSICLE: a, ARTICULAR 

 CAVITIES; d, THE DOUBLE DORSAL SHIELDS; I, LATERAL 



ARM PLATES. 



FIG. 34. SYNGNATHS OF OPHIURA CIUARIS. 

 AFTER MULLER FROM GREGORY. ;, JAW; 

 m.f., MOUTHFRAME; n, g., GROOVE FOR CIR- 



CUMCESOPHAGEAL NERVE-RING; p. d., PORE 

 AND DEPRESSION FOR ORAL TENTACLE. 



before the Triassic, since which time they occur more and more com- 

 monly. In the present oceanic waters they are popularly known as 

 sand-stars, brittle-stars, branching-stars, or basket starfish. They 

 range from shallow and estuarine waters to abyssal depths. Typical 

 Ophiuroidea differ from typical Asteroidea in having the arms sharply 

 marked off from the disk as appendages, and in the absence of 

 grooves along the actinal side of the arms. This means that the body 

 cavity which in the Asteroidea extends out into the rays is restricted 

 in the Ophiuroidea to the disk. 



The subclass Ophiuroidea is defined by Schondorf as follows: 

 "Ainbulacral water-vascular system situated in a small groove 

 at the base of the ray ossicles, and ventrally covered by a single 

 column of ventral shields. From the radial canal outside of the 

 ambulacrals arise simple lateral branches that never have ampullae, 

 as a rule curve upward, pass into and through the substance of the 

 ossicles, and finally open out laterally between the ventral and side 

 shields as the ambulacral podia. Ambulacrals opposite, each right 

 and left piece coossified into a vertebra with complicated articular 

 surfaces [see figs. 34-36]. Adambulacrals transformed into lateral 

 shields. Vertebrae dorsally covered by a single column of dorsal 

 shields." Disk circular in outline, lt without marginal plates, and 

 sharply separated from the rays, that as a rule are rounded. There 

 is no typical madreporic plate. One of the ventrally situated mouth 

 shields serves as madreporite" (1910a: 246). 



