352 A. E. Yerrill — JVbrth Americmi Ophinroidea. 



five or six, short, very thorny. Distal edge of under arm-plates 

 bent downward. 



dd. — Radial shields well apart, with intervening rows of scales. 



f. — Upper arm-plates not separated by the side plates. Two tenta- 

 cle-scales on first joint. 



O. Sarsii (Lym.). Radial shields rather small, far apart. Disk- 

 scales with small conical spinules. Arm-spines seven or eight, very 

 thorny. Tentacle-scale lobate. A stout oral tentacle-scale. 



^. — Upper arm-plates separated by the side plates. Disk-scales 

 coarse; mai'ginal ones larger. 



O. chelys (Lym.) Radial shields narrow, sunken. Disk with 

 small conical spinules. Arm-spines six, hollow, very thorny. One 

 tentacle-scale. 



AAA. — Ophiomitrella Ver., 1899a, p. 39. (See p. 343.) Radial shields 

 small, wide apart, naked. Disk-scales all nearly alike, not 

 very large, outlines easily visible. One tooth-papilla. Oral 

 shields join the side arm-plates. Interradial margins of 

 the disk not deeply emarginate and without large scales. 

 Disk bearing scattered granules or stumps. 



g. — Arm-spines serrulate, the rows not approximate dorsally. 



0. glohidifera (Koehler, '95). Arm-spines five. Disk-granules 

 glassy, spherical. Europe, 1700 meters. 



0. cordifera (Koehler^ '96). Arm-spines six or seven. Disk with 

 small capitate granules. Azores, 1143 meters. 



gg. — Arm-spines thorny and glassy, the rows approximate dorsally. 

 A special outer oral tentacle-scale on the first under arm- 

 plate. 



0. loBvipellis (Lym., as Ophiacantha). Seven or eight spines. 

 Disk-scales naked or sparsely granulated ; all small. Disk pentag- 

 onal. (See also page 343.) 



0. cornuta (Lym., as Ophiacantha). Spines eight. Disk rounded, 

 bearing small thorny stumps; a larger scale between the radial 

 shields. (See p. 339, note.) 



AAAA. — Radial shields very large and in contact, covering most 

 of the disk. Oral shields small, triquetral, not joining the 

 side arm-plates ; adorals large and broad. Arm-spines 

 smooth or nearly so, the rows not approximate dorsally. 



