VAN NAME: SIMPLE ASCIDIANS. 
553 
Mantle thin and musculature comparatively weak, although the 
same layers are present as in T. rusticum. 
Tentacles comparatively few, as in that species. 
Dorsal tubercle inconspicuous, often difficult to distinguish, and 
of unsymmetrical form, (.'-shaped or horseshoe-shaped with one horn 
inrolled, at least in some individuals. 
Dorsal lamina plain-edged and rather narrow. 
Branchial sac with four folds which are narrow in comparison with 
intervening spaces. The first two folds are the widest, the last one 
Text-fig. .31. — Tethyum atlanticum, sp. nov. X 1.4. 
narrowest of all. Transverse vessels and stigmata less regular in their 
arrangement than in the other New England species of this genus. 
At rather wide but not very regular intervals the sac is crossed by 
fairly stout transverse vessels. In the intervals between them are 
from 4 to 12 slender vessels which are often more or less irregular in 
their course and sometimes vary much in size among themselves, 
in other cases, however, are nearly equal. These narrow vessels 
separate the transverse rows of stigmata. They often show some 
tendency to converge toward that point on each fold which lies midway 
in the interval marked off by the large transverse vessels, and the 
stigmata conform more or less to this oblique direction of the vessels 
so that an appearance suggestive of the curved rows of stigmata 
characteristic of the Caesiridae results. This condition prevails, 
however, only on the folds, and close to their bases. On most of the 
wide interspaces the stigmata form rows which are regular except for 
occasional anastomoses or forking of the slender vessels above de- 
scribed. Very slender transverse vessels crossing without interrupt- 
ing the stigmata are present only here and there where the stigmata 
