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



7niis wurdemani; with which, however, they share the peculiarity of having, on the 

 interbrachial sides of the mouth frames, a series of deep furrows for the attachment of 

 the exterior mouth frame muscle. The nerve-ring is scarcely covered by the linear, 

 narrow, peristomial plate, which is in two pieces. A good example of detaU of finish 

 is found in the mouth angle, as seen in profile (fig. 13). There are the mouth 

 papillae (d) an even row ; and above them, in a close clump, the tooth papillaj {d') ; and, 

 stUl above them, the teeth {d") which have a quasi-enamelled grinding end. They are 

 supported by a well-marked jaw-plate (e) which has little pits above and cross furrows 

 below, for the insertion of the minute tooth, and tooth papiUae muscles. Outside this 

 are the sockets of the mouth tentacles, whereof the upper" one has a fixed scale, and the 

 lower may be said to have the^outermost mouth papilla as its scale. Outside these, 

 again, may be seen the wing of the mouth frames (/), whose opposite face bears the 

 muscle furrows already referred to. 



Some good specific characters may be found in the internal structure. Thus Ophio- 

 coma cethiops is distinguished from Ophiocoma riisei and Ophiocoma echinata by a finer 

 scaling of the central disk (six in the length of 1 mm.), and by larger supplementary scales 

 to the radial shields. Ophiocoma erinaceus has the corresponding disk scaling much coarser 

 (three in 1 mm.) than in Ophiocoma scolopendrina (five in 1 mm.), and the jaw cover of 

 the latter, though narrow, is thick and weU marked. Ophiocoma piimila, which leads a 

 group with long arms and a more delicate structure, has large radial shields without the 

 usual projecting corners, while their radiating rows of thick, ovei'lappiug scales are 

 almost wanting. The marginal scale belt, however, is well developed. OiMocoma p)ica has 

 an even but coarse scaling (two in 1 mm. near centre of disk) ; the radial shields, narrower 

 than in other species, are continued towards the centre by a narrow row of supplementary 

 scales ; the genital plate is prolonged, by upright scales, to the mouth shield. 



On the borders of this genus lie Ophiocoma papillosa and the old species Ophiocoma 

 nigra. The former I should doubtless refer to Ophiopteris were I sure that it 

 truly corresponded with its type-species Ophiocoma antipodum. It difi"ers from 

 Ophiocoma (1) in having only three narrow mouth papillae on each side, and more 

 numerous tooth papillae ; (2) in having small scale spines overlapping the base of the 

 upper arm spine [but this is found in Ophiocoma canaliculata'] ; (3) Ophiocoma 

 papillosa (and presumal:)ly Ophiocoma antipodum) has papillose tentacles ; (4) and almost 

 no striations for muscle attachment on the interbrachial sides of the mouth frame wings ; 

 (5) in well developed jaw-covers, which, however, are not larger than in Ophiocoma nigra. 

 Otherwise, the inner structure, as well as the outer in Ophiocoma papillosa is the same 

 as in Ophiocoma. As to Ophiocoma nigyxt it difiiers from the genus in its large and 

 thick peristomial plate, and in having hollow arm spines, which last peculiarity draws it 

 towards Ophioconis. Its relations to Ophiocoma miliaria and Ophiocoma antarctica 

 will be shown by the following table : — 



