EEPOET OX THE TUXICATA. 
257 
]\Iilne-Ed wards/ in 1841, formed the genus Leptoclinum as an addition to Savigny’s 
two genera, since he found that several species of Didemnidse which he studied possessed 
common cloacal cavities which received the atrial apertures of the various Ascidiozooids, a 
character which Savigny denied to both Didemnum and Eucoelium, although he plainly 
figures the cloacal apertures in the case of the latter genus. The three genera recognised, 
by Milne-Edwards, Didemnum, Eucoelium, and Leptoclinum, form the unistellar division 
of his Didemniens, the second great group of Compound Ascidians according to his 
scheme of classification. The genus Lissoclinum, founded by Verrill^ in 1871, requires 
to be re-examined and fully described before it can be referred to its proper position 
with certainty. Two species have been placed in the genus, Lissoclinum aureum and 
Lissoclinum tenerum, both from the eastern coast of North America. 
Giard,^ in 1872, definitely established the family'^ Didemnidse as a group including 
the three genera Didemnum, Eucoelium, and Leptoclinum, and distinguished by the 
possession of calcareous sj)icules in the common test from their nearest allies the 
Diplosomidse. He points out that the cloacal cavities are exceedingly difficult to 
distinguish in preserved specimens, and that, in all probability, they were present 
in Savigny’s specimens, although not detected by tliat investigator. Consequently 
iMilne-Edwards’ ground for the formation of the genus Leptoclinum does not 
really exist, since all the Didemnidae have common cloacal cavities. But, as Giard 
points out, all the species of Didemnidse examined by JMilne -Edwards are characterised 
by their habit of forming very thin crusts, much thinner than those of Didemnum, 
and consequently the name Leptoclinum may still be retained for them, although the 
distinguishing feature of the genus is not what its founder believed it to be. Giard 
separates Eucoelium from Didemnum on account of the transparency of the mantle, the 
obtuse and often rudimentary lobes round the branchial aperture, the length of the 
rectum, and the swelliog on the intestine, all of these being characters which are clearly 
shown in Savigny’s excellent figures. Eucoelium he distinguishes, on the other hand, 
from Leptoclinum by the thickness of the common test, the presence of the swelling 
on the intestine, and the form of the cloacal aperture, which is slit-like (as in 
Didemnum) in place of being open and wide as it is in Leptoclinum. 
Although Della Valle recognised the genus Leptoclinum in 1877,^ and described three 
new species, he appears to have afterwards abandoned it, as in his later work, in 1881,® he 
divides his section Didemnidi, which is equivalent to the present family, into two genera : 
— Didemnum (or Trididemnum), in which there are three rows of stigmata in the 
1 Observations snr les Ascidies Composdes, &c. 
2 Amer. Journ. Sci. and Arts, ser. 3, vol. i. No. 6, p. 443. 
® Eecberclies sur les Synascidies, Archives d. Zool. expdr., t. i. p. G44. 
* He called it a “ Tribe ” (Tribus I.), loc. cit, p. 644. 
^ Contribuzioni alia Storia Naturale delle Ascidie Composte del Golfo di Napoli, p. 45. 
® Nuovi Contribuzioni alia Storia Naturale delle Ascidie Composte del Golfo di Napoli, p. 50. 
(zool. CHALL. exp. — PART XXXVIII. — 1886.) Pp 3.3 
