KEPORT O^r THE TIJNICATA. 
267 
This genus was first instituted by H. Milne-Edwards under the impression that 
Savigny’s Didemnidse has no common cloacal cavities, while the species which he found 
living on the French coast had. As all the members of the family probably agree in 
having more or less well-developed common cloacal cavities and apertures, Milne- 
Edwards’ genus was really unnecessary, but it has been retained for a section of the old 
genus Didemnum, and now includes those species which form thin incrusting colonies, with 
a hard test stiffened by the presence of numerous calcareous spicules. In using the name 
in this sense von Drasche considers it as forming only a subgenus with Dideninoides as 
its companion group, but I feel convinced that Leptoclinum and Didemnoides are as 
distinct from one another as either is from Didemnum, and therefore I regard the three 
groups as being genera of equal rank. 
In Leptoclinum the Ascidiozooids have the atrial apertures placed near the anterior 
end of the body, and usually provided vdth long atrial languets. They never have simple 
atrial siphons like those of Didemnum. 
The branchial sac in Leptoclinum has usually four rows of stigmata, but this is not 
an invariable characteristic, as in Leptoclinum tliomsoni I have found that some of 
the Ascidiozooids in the colony have four rows, while one at least has only three, and in 
Leptoclinum propinqiium there are apparently only three rows of stigmata, and yet, as 
I shall point out in the description of that form, I cannot regard it as being a species of 
Didemnum. On the other hand, in Leptoclinum edwardsi five rows of stigmata were 
found in one of the Ascidiozooids examined, thus showing an approach to the condition 
characteristic of Eucoelium. 
The relations of the alimentary canal to the branchial sac may vary considerably in 
the genus. In most cases there is a distinct abdomen which extends beyond the thorax 
posteriorly, but in Leptoclinum moseleyi the alimentary canal lies on the dorsal edge of 
the branchial sac, and the body of the Ascidiozooid is short. This is probably a modifi- 
cation produced by the thin condition of the colony. 
A large number of species of Leptoclinum have been described, chiefly by Milne- 
Edwards, Giard, Della Valle, and von Drasche. It is extremely difficult to distinguish 
some of these species, especially in the case of spirit specimens, where the natural colour 
is lost. I believe, however, that I am right in considering that all the Challenger 
specimens of this genus belong to species previously unknown to science, with the excep- 
tion of those which I have referred to Leptoclinum albidum, Verrill, and its variety 
lutecium. It is possible that Leptoclinum jachsoni and two or three of the other more 
massive species should be removed from this genus and placed under Diplosomoides. 
The various species and varieties of Leptoclinum in the collection may be distinguished 
shortly by the following characters : — 
