BRACHIOPODA—THOMSON. I] 
SUPERFAMILY RHYNCON KLLACKA ScuucueEert. 
Genus Hemiriyris d’Orbingy, 1847. 
Genotype ANOMIA PSITTACEA Gmelin. 
HEMITHYRIS STRIATA sp. nov. 
(Plate XVI, figs. 30, 31, 32, 45.) 
Habitat—Station 11, off Shackleton Glacier (Davis Sea), 358 fathoms, 31st 
January, 1914. Sea bottom, ooze. 
There is from Station 11 a single ventral valve of a Hemithyris which must be 
referred to a new species. All trace of the animal had disappeared. ‘The shell is of 
a dull white colour with a nacreous interior. It is roundly triangular in shape, with an 
acute apex, is lightly and regularly convex without fold or sinus, and the margins of 
the valves are in one plane. The surface is ornamented with a few strongly-marked lines 
of growth and very numerous fine radial strise of somewhat uneven breadth, increasing 
in number towards the margin chiefly by intercalation, but occasionally by bifurcation. 
The beak is quite short, sub-erect, and possesses distinct beak ridges. ‘The foramen 
may be described as hypothyrid, but a ventralwards movement has commenced and 
has just destroyed the apex, about 1 mm. of which has been removed. The narrow 
delthyrium is partially closed by lateral deltidial plates which pass ventrally into a 
short anteriorly excavate pedicle collar. The hinge teeth are close, and are supported 
by dental plates, which incline towards one another in the ventral direction and the 
free margins of which are arcuate anteriorly. Muscular impressions a dull white, small, 
not extending far forward. There is no trace of a median septum in the beak. Shell 
substance imperforate; the imbricated structure is easily visible on the interior by 
means of a lens. ‘The shell is very thin, and the radial striation shows on the interior 
by transparence. The dimensions of the valve are-—length 17-5 mm., breadth 18 mm., 
thickness 4:75 mm. 
Three species of rhynconellids are known from Antarctic waters, viz., Rhynconella 
racovitzae Joubin (1901), R. gerlachei Joubin and Hemithyris sp. Jackson (1912). The 
first of these, Rhynconella racovitzae, was compared by Joubin to R. cornea Fischer, 
but it does not seem probable that these species are closely related. Joubin pointed 
out a difference in the course of the anterior commissure, viz., that the re-entrant angle 
is on the ventral valve in R. cornea, and on the dorsal valve in R. racovitzae, or in other 
words that R. cornea is incipiently ventrally uniplicate and R. racovitzae dorsally 
uniplicate.* Fischer and Oehlert’s figures (1891) seem to show, however, that &. cornea 
possesses a faint sinus in each valve, and that the folding is really of the Cincta type. 
This suggests the possibility that R. cornea may belong to the genus Frieleca, in which 
a similar tendency to a folding of the Cincta type is evident,} and the cardinalia certainly 
seem to show.an approach to the type described by Dall in Frieleia halli, although the 
hinge plates do not quite unite above the septum. Hemithyris craneana Dall is another 
* Cf, Thomson, 1915, No. 1 for terminology. } Cf. Thomson, 1915, No. 3, 
