EEPOET THE TUHICATA. 
323 
Branchial Sac large and well developed. Folds sometimes present. Internal 
longitudinal bars strong, and fairly numerous. 
Dorsal Lamina in the form of a plain membrane. 
Tentacles numerous. 
Alimentary Canal usually placed alongside the branchial sac, rarely extending 
beyond it posteriorly. 
Reproductive Organs in the form of polycarps attached to or imbedded in the 
mantle, and projecting into the peribranchial cavity. 
Gemmation effected by means of the vessels in the common test (?). 
I form this family for a very interesting little group of Ascidians, the position of 
which is difficult to determine. I regard them as Compound Ascidians which are allied 
to the Cynthiidse amongst Simple Ascidians, and have been evolved from the subfamily 
Styelinae.^ Various previously described forms must be placed here along with the new 
Challenger species. The history of the family is as follows ; — 
In 1850 Dr. Victor Cams, in a paper on the Zoology of the Scilly Isles,^ described 
the genus Thylacium. which he considered as a Social Ascidian allied to Clavelina. The 
individual animals in his species, Thylacium sylvani, were connected by a common 
fleshy base from which they projected upwards, the body was divided into an abdomen 
and a thorax, and both apertures were four-lobed. Car us considered that in Thylacium 
reproduction was probably effected by gemmation as well as sexually, and he placed the 
Cynthia aggregata of Forbes and Hanley, which he also regarded as capable of repro- 
ducing by gemmation, in his new genus under the name of Thylacium aggregatum. 
In 1863 Alder ^ gave a definition of Thylacium, Cams, and described two new species, 
Thylacium normani, which seems to be allied to the form described by Cams, and 
Thylacium variegatum, which differs from the other two in having the Ascidiozooids 
depressed and scarcely projecting from the surface of the colony. Alder does not 
specially mention the condition of the abdomen in this species, but I think from his 
general description of the body that the abdomen cannot be distinct from the thorax, 
and in that case this species ought not to remain in the genus Thylacium. I should be 
inclined then to remove Alder’s Thylacium variegatum from the genus Thylacium and 
place it in Giard’s genus Synstyela. 
In 1868 Dr. J. E. Gray briefly described,^ and figured in a woodcut, a new form which 
he regarded as a Social Ascidian and named Oculinaria austrcdis. It was found at Fre- 
mantle in Western Australia, and formed an erect elongated colony composed of a massive 
test in which rounded Ascidiozooids were imbedded. The test contains imbedded sand- 
grains. There can be little doubt that Oculinaria belongs to this family, but whether it is 
1 See Summary and General Eemarks at the end of this Eeport. 
2 Proceedings of the Ashmolean Society, vol. ii. p. 264, Oxford, 1843-52. 
® Ann. <md Mag. Nat. Hist., ser. 3, vol. xi. p. 152. ^ Proe. Zool. Soc. Lond., vol. for 1868, p. 564. 
I 
