310 
THE VOYAGE OF H.M.S. CHALLENGEE. 
neural canal lies below the latter. The oblique muscles, though slender, are more 
conspicuous than in the previous examples. The longitudinal ventral muscles seem to 
be pierced by certain fibres passing from the dorsal arch to the outer side of the 
longitudinal muscles of the region. The structure, though closely allied, is thus not 
identical with the form just described. 
Nothria somhreriana, n. sp. (PI. XL. figs. 5-7 ; PI. XXIa. figs. 19-23). 
Habitat. — Dredged off Sombrero and St. Thomas, West Indies, in 470 and 390 
fathoms, 1873. 
The larger specimen (in two pieces) measures about 43 mm., and has a breadth 
across the feet anteriorly of barely 2 mm. 
The head (PI. XL. fig. 6) is marked by brownish pigment, which extends into most 
of the tentacles. The median tentacle is moderately elongated and tapered, and slightly 
exceeds the adjoining pair in length. The latter have a similar shape and appearance. 
Both sets rise from a base possessing a few rings. The basal segment of the outer pair 
is smaller, but is similarly ringed. The organs themselves are only about a fourth the 
length of the foregoing inner tentacles, are less tapered, proportionally thicker, and 
more curved. The tentacular cirri are short and tapering. The palpi differ from those 
of Nothria conchylega in being more globular. No eyes are present. 
The dental apparatus (Fig. 66) is comparatively pale in the smaller forms, more 
dusky in the larger, the chief coloration being a dark transverse line separating the 
posterior (spathulate) processes from the maxillse, and a symmetrical touch of the same 
hue on the great plates a little in front of the fork of the latter. The maxillae have 
the posterior part of the blade flattened, the anterior diminished somewhat suddenly ; 
while the latter region is tinted brownish. The posterior appendages are somewhat 
triangular, and considerably shorter than in Nothria conchylega. The left great dental 
plate has six teeth, ten being present in the form just mentioned ; the size, prominence, 
and separation of the first three being remarkable, and a feature apparently characteristic 
of the series (Fig. 68), indeed at first sight the left side seems to be furnished with two 
maxiUae, so prominent is the elongated anterior hook of this plate. In the large 
examples there are seven or eight teeth. The right plate has eight teeth. The left 
lateral paired plate presents seven or eight teeth, and the accessory has a single blunt 
tooth. The unpaired of the same side has eight teeth. The right lateral plate has 
about ten teeth. The ventral dental surface of the mandibles (Fig. 67) has a prominent 
ledge on each side behind the crown. The corresponding ledges form a broad V, with 
a pigment-line round the posterior margin (Fig. 69). The cutting edge or crown is 
armed with two or three prominent denticulations, sometimes symmetrically disposed. 
A longitudinal line of dark pigment occurs on the dorsal surface on each side of the 
