REVISION OF PALEOZOIC STELLEEOIDEA. 



269 



Genoholotype. 0. flexilis Meek and Worthen. 



For generic description, see under 0. flexilis. 



Remarks ly Gregory.- "This interesting genus has hitherto been 

 placed among the Euryalidse, of which it has been regarded as the best 

 known fossil representative. As Prof. Bell, however, has remarked, 

 Meek and Worthen's clear figures of the vertebral ossicles show that 

 the articular surfaces are Strep tospondy line and not Cladophiuroid." 



Schondorf in 1909 restudied Ony chaster hi detail, and came to the 

 following conclusions : 



u Ony chaster flexilis shows plainly in the structure of its arm ossicles 

 that it belongs to the Ophiuroidea. As in this subclass, so also in 

 Onychaster the arm ossicles originated from two halves (ambulacrals) 

 that grew tightly together. When compared with living Ophiurids, this 



40 



41 



FIGS. 37-41 .ONYCHASTER FLEXILIS. AFTER SCHONDORF. 37 AND 38, PROXIMAL VERTEBRA SEEN FROM 



THE ABORAL AND ADORAL SURFACES. 39, VENTRAL VIEW OF TWO PROXIMAL VERTEBRA, ORIENTED 

 WITH THE ABORAL SIDE UP AND THE ADORAL DOWN. 40, SIDE VIEW OF SAME. ADORAL SIDE ON LEFT, 

 ABORAL ON RIGHT. 41, VENTRAL VIEW OF SEVERAL SUCCESSIVE VERTEBRA WITH THE ABORAL SIDE 

 AT THE TOP. O, UPPER LATERAL ARTICULAR KNOB; O', SOCKET FOR KNOB OF ADORAL SIDE; C, INSERTION 

 FIELD FOR THE UPPER, AND 0, FOR THE LOWER INTERMEDIARY VERTEBRAL MUSCLES; gr, MEDIAN UN- 

 PAIRED DEPRESSION; gr\, SMALL DEPRESSION FOR THE KNOB, fci, OF THE ADORAL SIDE; gr, LARGE DE- 

 PRESSION OF THE CENTRAL AREA OF VERTEBRA; TO, BARRIER SEPARATING THE UPPER AND LOWER 

 INTERMEDIARY VERTEBRAL MUSCLES; T, LATERAL GROOVE (? FOR LATERAL BRANCHES OF WATER- 

 VASCULAR SYSTEM); S, VENTRAL PROJECTION OF THE LATERAL KNOBS OF ADORAL SIDE; t AND t' , UNPAIRED 

 MEDIAN ARTICULAR KNOB AND SOCKET. 



amalgamation of the two arm ossicles in Onychaster is still plainly to be 

 seen. The equivalent skeleton of the adambulacrals in the asterids is 

 not yet completely separated from the arm ossicles nor developed into 

 independent side or lateral shields ; on the contrary, they still remain 

 attached and in articulation with the arm ossicles on their ventral outer 

 margin. On their somewhat fluted outer margin they bear a few spines. 

 The arm ossicles are not externally naked but are covered by smaller 

 additional plates that bear flat calcareous particles. Individual large 

 dorsal shields do not appear to be present, for on the large Berlin speci- 

 men the arm ossicles lie directly beneath the small flat dorsal plates. 

 Only in the distal part of the rays does one observe some rows of 



