REVISION OF PALEOZOIC STELLEROIDEA. 



269 



Genoholotyi^e. — O.Jlexilis Mook and Worthcn. 



For generic description, see imder O.Jlexilis. 



RemarTcs hy Gregory.- -"This interesting genus has hitherto been 

 placed among the Eiiryalida3, of which it has been regarded as the best 

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

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

 the articular surfaces are StreptospondyluK^ and not Cladophiuroid." 



Schoudorf in 1909 restudied Onycliaster in detail, and came to the 

 folio vving conclusions : 



" Ony chaster jlexilis shows plainly in the structm'o of its arm ossicles 

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

 OnijcTiaster the arm ossicles originated from two halves (ambulacrals) 

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



Figs. 37-41.— Ontchasterflexilis. After SchOndorf. 37 and 3S, Proximal vertebr.v seen from 

 the aboral and adoral surfaces. 3'), ventral view of two proximal vertebr.'e, oriented 



■\\nTH THE ABOKAL SIDE UP AND THE ADORAL DOWN. 40, SiDE VIEW OF SAME. -XDORAL SIDE ON LEFT, 

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

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

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

 PAIRED depression; qtx, small depression for the knob, fci, of the adoral side; j/r;, large de- 

 pression OF THE CENTRAL AREA OF VERTEBRA; 7«, BARRIER SEPARATING THE UPPERAND LOWER 

 intermediary VERTEBRAL MUSCLES; r, 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 boar flat calcareous particles. Individual large 

 dorsal shields do not appear to be present, for on the largo 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 



