174 ECHINODERMS OF THE BRITISH ISLES 



3. Arm spines generally 7, increasing very gradually in length from 



the first to the fifth, then decreasing. Disk with spines or 

 spinelets, not articulated to distinct tubercles 



1. O.frarjilis (Abildg.) 

 Arm spines generally 6, increasing very rapidly in length from 

 first to fourth or fifth, the upper one being very much shorter. 

 Disk with strongly serrate spines, articulated to distinct 

 tubercles. (Koehler, " fichinodermes," Faune de France, p. 72, 

 Fig. 47) . . . . .0. quinquemacidata {D . Ch.) 



4. Dorsal plates of arms carrying small spines 



2. O. LiXtkeni Wyv. Thorns. 



Dorsal plates of arms not carrying spines (" Talisman " Oph., 



p. 296, PI. xix. 22-26) . . . . O. indigna Koehler 



1. Ophiothrix fragilis ( Abildgaard) . (Fig. 98.) 



(Syn. Ophiothrix alopecurus Miill. and Trosch. ; 0. Ferussacii 

 Miill. and Trosch. ; 0. Bammelsbergii Miill. and. Trosch. ; 

 0. rubra Ljungman ; ^ Ophiocoma rosula Forbes ; 0. minuta 

 Forbes). 



Disk covered with short spinelets or longer, slender spines, 

 mostly both kinds intermingled. Oral side of disk only Avith 

 short spinelets, the proximal part naked. Radial shields large, 

 generally naked. Dorsal plates of arms naked, with a more 

 or less distinct longitudinal keel ; the outer edge more or less 

 distinctly produced, often with a small elevated, whitish knob. 

 Ventral plates almost rectangular, with a small depression proxi- 

 mally, the outer edge slightly concave. One small tentacle scale. 

 Generally seven arm spines, increasing very gradually in length 

 from the very small lowermost one to the fifth or sixth, then 

 decreasing again. Disk often strongly projecting between the 

 arms (depending on the state of development of the genital organs). 



Reaches a size of ca. 20 mm. diameter of disk. Arms ca. five 

 times the diameter of the disk. Colour in life exceedingly variable, 

 from a uniform dark violet to almost white or reddish, spotted, 

 often with arms banded. 



This species is subject to very great variation, especially as 

 regards the spines on the disk, the shape of the radial shields, and 

 the length of the arm spines. This accounts for a number of 

 " species " which can at most be regarded as varieties. Koehler, 

 who has recently undertaken a detailed study of all these forms, 

 distinguishes the following varieties : Var. echinata Delle Chiaje 

 (non : Miiller and Troschel), var. pentaphyllum (Ljungman), 



1 The author has had the opportunity of studying the type -specimen 

 of Ljungman's Ophiothrix rubra, and finds it to be identical with O. fragilis. 



