338 
ments are often very difficult to distinguish. In some of the spec- 
imens the alae are rather well developed, but the radii are always 
rudimentary. The aperture is not large, although irregular owing 
to corrosion. The colour (in alcohol) is greyish-white. 
The apparition corresponds* to the figures given by Darwin 
(1853) of corroded specimens of Tetraclita purpurascens, and the spec- 
imens on the whole so nearly 
agree with Darwin’s description 
of this species that they must 
be referred to it. Nevertheless 
they also exhibit so marked dif¬ 
ferences that they must be taken 
as representatives of a special 
form. 
One of the characters em- 
phasized by Darwin was the 
unusually long scutum of the 
species. In the present specim¬ 
ens (Fig. 71) the scuta are much 
shorter, their basal margin being 
of the same length as the tergal 
margin; hence the name brevi- 
scutum. In other respects the 
opercular plates fairly well agree 
with Darwin’s description when 
heed is paid to the corrosion of the apical parts, especially of the 
tergum. 
Dealing with the mouth parts Darwin says that „the lab¬ 
rum is deeply notched and apparently destitute of teeth on the 
crest“. The notch is in the present specimens rather shallow, and 
on the oral side the labrum is armed with a single row of small, 
rounded denticles at the notch; the denticles are very small, and 
only with difficulty observed. 
The rudimentary fourth tooth of the man di ble is emphasized 
by Darwin as a distinguishing feature of the species; the char¬ 
acter is alo observed in the present specimens (Fig. 72), and separ¬ 
ates them from Tetraclita squamosa, with which Tetraclita purpura¬ 
scens is closely related. Characteristic of Tetraclita purpurascens is 
Fig. 71. Tetraclita purpurasceus f. brevi- 
scutum from Port Ross. a and b external 
view of tergum and scutum, c and d in¬ 
ternat side of scutum and tergum. [X 4]. 
