VAN NAME: SIMPLE ASCIDIANS. 565 
Tethyum mortenseni Hartmeyer. 
PI. 61, fig. 98-100; text-fig. 34. 
1912. Tethyum morlenseni Hartmeyer, Vidensk. Meddel. Naturhist. For. 
Kjobenhavn, vol. 63, p. 268, text-fig. 3. 
The largest specimen (from Station 184, see below) is nearly globu- 
lar except for a flattened area on the ventral surface by which it was 
attached. Diameter 15 mm. Apertures on dorsal surface, small, 
four-lobed, 6 mm. apart; scarcely raised above the general surface 
in the preserved specimen. Test (except attached area) very thin, 
Text-flg. 34. — Tethyum mortenseni Hartmeyer. X 2.4. 
but tough. Surface not smooth, yet practically free from incrusting 
material and without any conspicuous folds or wrinkles. Color of 
preserved specimens yellowish. 
The two other specimens (from Station 2506) are slightly smaller 
(13.5 mm. and 11 mm. in greatest diameter, apertures 5 mm. apart), 
somewhat darker colored and a little more compressed laterally, but 
otherwise very similar. 
Mantle thin and musculature slight. 
Tentacles arranged with a considerable degree of regularity. There 
are about 28 larger ones which, on some parts of the circle at least, 
show some alternation in size and must represent two orders, and be- 
tween these there are one or more smaller ones in most of the intervals. 
Dorsal tubercle in the largest specimen horseshoe-shaped, the horns 
coming almost together; open interval directed obliquely forward 
and to the right. In one of the others, there appears to be no open 
interval at all, the horns coming completely together at the anterior 
end. On the third specimen, the open interval, if any (that which is 
present may be only an accidental break) is decidedly toward the left. 
Dorsal lamina rather broad, plain-edged. 
