BEVISION O!^ PALEOZOIC STELLEKOIDEA. 



267 



Formation and locality. — OkaAV 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 tlie older Paleozoic, may have 

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



Fig. 33. — OpinoTEEE.?is. After Bell from Gregory. 

 Aboral surface of an arm ossicle: a, articular 

 CA^^TIEs; d, the double dorsal shields; /, lateral 

 arm plates. 



Fig. 34. — Syngnaths of Ophiura cill\ris. 

 After MOller from Gregory, j, jaw; 

 TO./., mouthframe; n. g., groove for cir- 



CUMCESOPUAGEAL nerve-ring; p.(/.,PORE 

 .\ND 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 stai"iish. They 

 range from shallow and cstuarine waters to abyssal depths. Typical 

 Opliiuroidea differ from tj'pical 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 Opliiuroidea is defined by Schondorf as follows: 

 ''iVmbulacral water-vascular system situated in a small groove 

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

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

 ambulacrals arise simple lateral branches that never have ampulla?, 

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

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

 shields as the ambulacra! podia. iVmbulacrals opposite, each right 

 and left piece coossified into a vertebra with compUcated articular 

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

 shields. Vertebrae dorsally covered by a single column of dorsal 

 shields." Disk circular in outUne, 'S\dthout 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). 



