CLASS II 



OPHIUROIDEA 



255 



and-socket joints. The arm-plates are move or less completely developed and the side 

 arm-plates may carry spines. Arms may be very short with relatively enormous 

 tuhe-feet, as is apparently the case in Eucladia. 



This order includes a number of Paleozoic forms and not a few Eecent 

 species. Important genera are Ophiurina Stiirtz, Devonian, with separate am- 

 bulacral ossicles and no ventral arm- 

 plates ; Lapworthura Gregory, Silurian, 

 with barely fused ambulacral ossicles, 

 and no ventral arm-plates ; Sympterura 

 Bather, Devonian, similar to Lap- 

 ivorthura, but with narrower rays and 

 spinulose disk ; and Eoluidia Stiirtz, 

 Devonian, with fused ossicles, and with 

 ventral arm -plates. The genus Ony- 

 chaster Meek and Worthen (Fig. 361), 

 Lower Carboniferous, has usually been 

 regarded as representative of the 



■1 Tji Tj -u i it, 1, i„ „£ Omieliaster flexilis M. and W. Liower (Jarbonit'erous : 



modern Euryallds, but the character of Crawlonlsville.I.id. (after Meek and Worthe,.). .4,In- 



the ambulacral ossicles necessitates its dividual of the natural size with roUed up arms; 



the dorsal covering of the central disk is removed, 

 inclusion in this order. The remarkable exposing; the mouth frame. £, Mouth frame enlarged, 

 o • 1 • n 7 7 • ^TT 111 viewed from above : C, Vertebral ossicle, enlarged. 



Silurian genera Lucla\iia Woodward, and 



Euthemon Sollas, are exceedingly diflficult to place, and their relation to other 



Ophiurans is problematical. 



Fig. 361. 

 Onyehaster flexilis M. and W. Lower Carboniferous ; 



Order 3. CLADOPHIUROIDA. 



Ophiuroidea in lohich the vertebral ossicles are complete and articulate with each 

 other by means of hourglass- shaped surfaces. The arms are often dichotomously 

 branched and lack regular series of arm-plates. 



This order includes a large number of Recent forms, those with branched 



Fig. 362. 



Aspidura loricata (Goldfuss). Muschelkalk ; Waschbach, Wiirtemberg. A, group of individuals of the 



natural size (after Quenstedt). B, Ventral aspect, enlarged (after Pohlig). 



arms {Astrophyton, Gorgonocephalus, Euryale) being known as " Basket-fish " or 

 " Sea-spiders." Fossil forms are rare, but certain Mesozoic remains of doubt- 

 ful position have been referred to the Recent genera Astrocnida and Euryale. 

 Onyehaster, the Streptophiuran referred to above, has peculiarities that suggest 



