24 OPHIURA OLIVACEA. 



the plates nearly an octagonal shape. Side arm-plates encroachmg less 

 than usual between lower arm-plates. Upper arm-plates broader than 

 long, bomidedj without and within, by very straight, clean lines ; inner 

 side shorter than outer ; outer corners well rounded ; three or four 

 plates enclosed by projections from disk, and modified in form accord- 

 ingly ; length to breadth (sixth plate), .9 : 1.8 ; about two thirds out on 

 arm, same general shape, but j^roportionately longer. Disk, except side 

 mouth-shields and a very few naked scales at base of arm, closely cov- 

 ered with very fine grains, about 170 to a square mm. Arm-spines six, 

 of equal length and size, someAvhat flattened, but little tapering, points 

 rounded ; length to that of under arm-plate, .5 : .6. Inside tentacle-scale 

 oblong, with straight sides and rounded point ; length^to that of under 

 arm-plate, .4 : .6 ; outside scale shorter and broader. Color, in alcohol : 

 above, uniform dark sap-green, without bands on the arms ; below, the 

 same, but lighter ; interbrachial spaces slightly mottled^ and of a darker 

 shade than arms. 



Variations. — The specimen above described is below the ordinary 

 size of well-grown individuals. A specimen with a disk 14.5°"° in diam- 

 eter had eight strongly flattened arm-spines, fifteen mouth-papilla?, and 

 mouth-shields longer than broad, and pretty regular heart-shaped ; disk 

 w^th about 130 grains to a square Tnm. The number of teeth varies 

 somewhat ; four seems to be most common. A specimen with disk 

 IG"'"' had arms 74™"-, and only seven arm-spines. The upper arm-spines 

 are often, in large specimens, a little longer than the lower. 



When alive, O. oUvacea is of a bright green color. It lives on sandy 

 bottom, among eel-grass, and is active in its motions (Ayres). It is, at 

 first sight, very like O. hrevispina., but is readily distinguished by more 

 flattened arm-spines, and by the form of the under arm-plates, which are 

 nearly square, with straight lateral sides, instead of hexagonal, with the 

 lateral sides decidedly re-enteringly curved. It is further remarkable 

 for uniformly colored arms without bands. Young or partly grown 

 specimens, however, are often undistinguishable from O. h7^evisj)ina. 

 From other species with equal spines and covered radial shields, it is 

 distinguished as follows : from 0. brevicaiida, by finer granulation and 

 longer arm-spines ; from O. guttata, hy whole upper arm-plates ; from 

 O. variegata, l)y shorter arm-spines ; and from O. Januarii, hy shorter 

 and broader arm-spines. 



There is in the Philadelphia Academy a specimen, collected on the 

 shores of Rhode Island by Prof A. D. Bache, which agrees entirely with 

 this species, except that it has radial shields. It is possible that the 

 grains have been rubbed off the radial shields, after the animal was dry. 

 Another specimen, in the Museum of Comparative Zoology at Cam- 

 bridge, has, outside four of the mouth-shields little supplementary 



