362 THE VOYAGE OF H.M.S. CHALLENGER. 
eighteen or twenty joints, the lowest of which are cuboidal. The next few, though still 
very thick, are much flattened laterally and gradually diminish in width, with the lateral 
edges of the dorsal surface raised and thickened. The terminal part of the pinnule 
consists of much smaller joints, and is more or less distinctly serrate. In the pinnules of 
the third and the next following distichals the dorsal surface of the two or three thick 
basal joints is rather broad, but the following joints diminish rapidly both in breadth and 
in thickness. Beyond the distichal axillaries all the pinnule-joints are longer than wide, 
with the exception of the first two, which are much expanded, and this character is very 
marked in all the following pinnules till about the level of the fourth axillaries. Beyond 
this pomt the second and third pinnule-joints more nearly resemble their successors, 
though traces of the expansion of the first joint are visible for some distance farther. 
The disk (so far as it is visible) is well protected by plates, both on its ventral surface 
and on its sides, right down to the hypozygal of the second brachial. The brachial 
ambulacra are but little above the narrow arm-groove, and are protected like those of the 
large lower pinnules by very irregular plates. The terminal pinnules have well defined 
and rather pointed side plates. 
Colour—a uniform dusky purple when fresh (Moseley); in spirit, ight brownish- 
white. 
Locality.—Station 214, February 10, 1875; off the Meangis Islands; lat. 4° 33’ N., 
long. 127° 6’ E.; 500 fathoms; blue mud; bottom temperature, 41°°8 F. Two specimens, 
with Myzostoma wyville-thomsoni, von Graff. 
Remarks.—This elegant little species is the smallest Metacrinus which I have yet 
seen, with the exception of Metacrinus nodosus; and it has many points of resemblance 
with that type, as will be explained later. Although the stem and cup are much less 
robust than in Metacrinus wyvillii, the number of arms is nearly half as large again as in 
that species, which rarely has an axillary beyond the palmars; while in Metacrinus 
costatus this is generally the case on four out of the eight tertiary arms, and there may 
be another axillary beyond the supra-palmar. The number of internodal joints is nearly 
the same in the two species, being rarely less than seven in Metacrinus costatus, though 
sometimes falling to five in Metacrinus wyvillii. But they are totally different in their 
form and external markings, as will be evident from a comparison of Pl. XLVII. 
fies. 1-4, and Pl. XLIX. figs. 3, 4. The nodal joints are also quite different in the two 
species. The cirrus-sockets of Metacrinus wyvillii (Pl. XLVII. figs. 1, 2) extend both 
upwards and downwards on to the supra- and infra-nodal joints beyond the articular 
facets, which occupy the whole height of the nodal joints. But, this is far from being the 
case in Metacrinus costatus; and the nodal joints therefore are less deeply incised than 
in Metacrinus wyvillii, while their angles are much sharper and more produced outwards 
(Pl. XLIX. figs. 3, 5). There is also a good deal of difference between the pinnules of 
the two types. Both those on the radials and distichals of Metacrinus wyvillii and those 
