MUSEUM OF COMPARATIVE ZOuLOGY. 245 



closeness of the various stumps and spines mentioned above ; there also was 

 sometimes a much elongated form of 49 (Fig. 55). Among the young, the 

 arm is from five to seven times the disk ; among the adult, from six and a 

 half to ten times. 



The blue species, already mentioned as living beside 0. quinquemaculata, 

 is that identified by Miiller & Troschel with A sterias echlnata Dell. Ch. One 

 of their originals at Berlin lias the disk 11 mm. and the arm about 40 mm. 

 The upper arm-plates are rhomboidal, overlapping, and with a slightly 

 thickened lobe without; length to breadth .9 : 1.2. There are nine short, 

 stout, little, tapering arm-spines, the longest 2.1 mm., and with a dozen 

 blunt, feeble thorns on each edge. The disk is evenly set with larger and 

 smaller trilid stumps (PI. II., Figs. 2, 3), with very few small cylindrical 

 spines (Fig. 1). This is as large a specimen as I have seen, for the species 

 is small ; it has a puffed disk, in which the radial shields' are somewhat 

 sunken, and arc therefore not conspicuous. The arms are always short ; in 

 five adult specimens the average of the arm to the disk was as 5 : 1. Already, 

 with a disk of 7 mm., the adult characters are taken on ; the upper arm- 

 plates are broader than long, as 1 : .G ; while a specimen of O. quiitquemacu- 

 lata, of the same size, had them of equal dimensions, .9 : .9. The arm-spines 

 vary little ; they are even, short, stout, and little tapering, and are from 

 seven to nine in number. The smallest specimen (Naples) had a disk of 

 2.5 mm., and the arm 13 mm.; the scaling was very distinct, each scale 

 usually bearing a slender, trifid stump (PI. II., Figs. 12, 13); on the upper 

 surface their character was the same, but some were simply forked ; on the 

 radial shields were a few similar but smaller stumps, which, in the adult, 

 wholly or nearly disappear. Another had a disk of 7 mm., which carried on 

 its upper surface, evenly set, two and three forked stumps (Figs. 5, 9) and a 

 few short spines (Fig. 4), there being, on the radial shields, some little trifid 

 ones (Fig. G). The lower interbrachial spaces were closely set with long 

 stumps and short spines (Figs. 7, 8) ; between the radial shields of the same 

 pair there was a single line of stumps. Another specimen — disk 10 mm. — 

 was evenly set with little forked stumps, covered with a thick envelope of 

 skin (Figs. 10, 11). An individual from Algeria had a few forked stumps so 

 elongated as more properly to lie called spines (Fig. 14), but the greater part 

 of the armature consisted of forms similar to G, 11, and 12. Four specimens 

 from the Adriatic and one from Egypt presented no new features. It will be 

 noted of this species : first, that it is small ; secondly, that the disk-stumps 

 are fine, and never have more than two or three thorns as a crown, while 

 longer spines are wanting, or very rare ; thirdly, that the arms are short, 

 being from three and a half to six times the disk. 



llelier* has recalled attention to the Adriatic species 0. alopecurus Mull. 



* Zooph. und Echinod. des Adriatischcn Moeres, 63, 1868. 



