370 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Family DIDEMNIDAE Venill, 1871. 
Genus Didemnum Savigny, 1816 [= Leptoclinum auct. plur.]. 
Colonv usually thin and expanded; sometimes thick and fleshy. 
Common test containing stellate spicules, usually in great abundance. 
Branchial aperture six-lobed; atrial aperture a plain opening without 
a languet. Four rows of stigmata in the branchial sac. Testis single 
or more or less completely two-parted. Proximal part of sperm duct 
coiled about the testis. 
Didcmnvm candidum Savigny, 1816, the type of the genus, has four 
rows of stigmata, and belongs to the group which has usually been 
called Leptoclinum Milne-Edwards, 1841. Therefore, according to 
the rule of priority Didemnum must replace Leptoclinum in the sense 
that the latter term has generally been used. 
In the above diagnosis of this genus, there are two points of difference 
from that given by Hartmeyer (1909, p. 1448). According to him, 
an atrial languet is present, and the testis is always undivided. Hart- 
meyer's classification makes no provision for the numerous forms of 
this group in which there is no atrial languet. The languet is, in fact, 
lacking in the type species of the genus, as Savigny's (1816) figure 
clearly shows. 
Owing to the numerous species in this family, and the difficulty 
of satisfactorily classifying them, a subdivision of the group into a 
larger number of genera than at present will tend to clearness rather 
than obscurity, and believing that the presence or absence of an atrial 
languet is a good generic distinction, the writer has in this paper 
adopted the course of limiting Didemnum [Leptoclinum] to species 
which, like Savigny's type, lack the languet. From Leptoclinides 
Bjerkan, 1905, Didemnum is sufficiently distinguished by lacking the 
posteriorly directed tubular atrial siphon, having only a simple aperture. 
Hartmeyer's diagnosis also disregards the fact that in this group of 
genera a two-parted testis is common. The writer does not consider 
this of importance as a generic character, but has so worded the 
diagnosis as not to exclude species so provided, and for the forms with 
an atrial languet, he has adopted, after some hesitation, the name 
Tetradidemnum Delia Valle, 1881 (see below). 
Two species of Leptoclinum were described from New England by 
Professor Verrill (1871a) — L. (dbidum and L. Iiiteolum, the latter 
