Mr. J. Alder on the British Tunicata. 167 



kindly forwarded to me by Mr. Peach, consists of a globular 

 mass of individuals of all ages and sizes, piled upon each other 

 so as to resemble a large fruit of the Rubus tribe. The extra- 

 neous substance to which they are attached is so completely 

 covered as not to be discernible; and the individuals themselves 

 adhere so closely that, at first sight, they appear to form one 

 compound animal. That this is not the case, however, may be 

 seen by a more minute inspection, when the line of junction 

 between each can generally be detected, and, with a little care, 

 an individual may be detached entire, showing no point of 

 organic junction with the rest. The young fix themselves on 

 all parts of the older ones, and in the spaces between them, so 

 that, in process of time, a globular mass such as here described 

 is the result. 



Genus Thylacium, Victor Cams. 



" Common base a broad fleshy structure supporting closely- 

 set individuals ; outer tunic coriaceous ; both orifices with four 

 lobes ; abdomen as long as the thorax." 



Dr. T. Victor Cams thus characterizes a genus established by 

 him in the ' Proceedings of the Ashmolean Society ' (vol. ii. 

 p. 266) for the reception of an Ascidian found in the Scilly 

 Islands. The connexion of the individuals of this genus by a 

 solid fleshy base has induced the learned author to include it in 

 the family Clavelinida. It has, however, a much closer affinity 

 with the AscidiacUe, especially with the two small gregarious 

 species of Cynthia ']ns,i described. Dr. Carus considers that this 

 genus is propagated by gemmation as well as by ova. Further 

 observations are desirable on this point. 



Thylacium Normani, n. sp. 



Body subclavate, rounded above and contracted a little below, 

 reddish, firmly fixed in groups to a common fleshy base. Aper- 

 tures rather large and prominent, set considerably apart at the 

 upper end, quadrate, or nearly circular when expanded, some- 

 times appearing as a single slit when closed. Test strongly 

 wrinkled or subtuberculated, rather smoother near the apertures. 

 Tentacular filaments linear, very slender. Branchial sac with 

 two or three (?) folds on each side. Height about two-tenths of 

 an inch. 



Mr. Norman, to whom I am indebted for the knowledge of 

 this species, found it studding the roof and sides of the cele- 

 brated Gouliot Caves, Isle of Sark. It is much smaller than the 

 T. Sylvani of Carus, of a difierent form, and appears to spread 

 more horizontally. 



