214 THE VOYAGE OF H.M.S. CHALLENGER. 



outer and inner faces of the arm bones are quite changed (figs. 6 and 7). The upper 

 portion is occupied by the narrow apophysis, or, in the inner face, by the deep slot, so 

 that the articulating umbo (l) and its corresponding hollow (4) are reduced to small 

 proportions. The lower muscle-field (w) is also small as compared with the upper (?</')• 

 Not less characteristic are the mouth angles with their long-crested, deeply-grooved upper 

 surface (f) nearly or quite destitute of a peristomial plate, the nerve being well protected 

 by its extremely deep canal (»). The jaws (c) are very high, but not long. Their height 

 (fig. 8) gives room for the great vertical oval of tooth papillae (d r ) and the numerous but 

 shorter teeth (d"). In shape the jaw plate is like a shoe hole perforated for the attaching 

 ligaments ; the wider end is uppermost and bears the tooth papillae. Outside this appears 

 the jaw, at whose upper end is attached, as usual, a fold of the stomach (st), and in whose 

 sides are the sockets from which issue the large fleshy mouth tentacles (rr). 



Ophiothrix is the Salmo of echinoderms ! Well defined and peculiar as a genus it 

 has a crowd of species, many of which are the despair of the specific zoologist. From the 

 internal skeleton some aid may be got in this direction. Thus Ophiothrix hirsuta has a thick 

 disk skin set with small separated scales, each bearing a thorn, or spine. Its young has 

 radial shields proportionately larger and more nearly approaching in the interbrachial 

 spaces. From it the kindred species Ophiothrix longipeda is well distinguished by a 

 generally lighter structure ; a narrower genital scale, and more slender genital plate ; 

 smaller radial shields having over twenty interbrachial radiating scale rows, instead 

 of eight to fourteen as in Ophiothrix hirsuta; and, finally, in having the outer horns 

 of the mouth frames shorter and less grooved. Ophiothrix trilineata stands near, 

 but has the disk scales large and few. Ophiothrix augulata and Ophiothrix orstedii 

 have a similar general structure ; but have a close, well-marked imbricated scaling, 

 with about seven radiating interbrachial rows between the radial shields. Their 

 young have proportionately smaller radial shields and the scales wider. Ophiothrix 

 spicidata is closely allied, with larger radial shields, however, and the outer horns 

 of the mouth frames much prolonged. Ophiothrix fragilis, 0. echinata, 0. penta- 

 phyllum, O. quinquemacidata, and 0. alopecurus have a common type. The first 

 free arm-bone has a little hollow lozenge on its upper surface (fig. 5), whde in most of 

 the species it has a thin, slightly grooved margin. Then the narrow brachial space 

 between the radial shields is filled by a line of long, thick scales. Finally, there is a 

 large space of wholly naked skin near the mouth shields. The specific differences which I 

 pointed out 1 between the large Ophiothrix fragilis of North Europe and the small Ophiothrix 

 echinata of the Mediterranean are confirmed. A young of Ophiothrix fragilis, had a disk 

 6 mm. in diameter, whose upper surface was almost wholly filled by contiguous radial shields, 

 while in Ophiothrix echinata (disk 8 mm.) there were as many as eight scale rows in 

 each interbrachial space, and the radial shields had the lobed margin of the adult. 



1 Bull. Mus. Comp. Zool., vol. iii., part 10, p. 245. 



