SIDNYCJM TUBBINATUM. 31 



system with a common excretory orifice in the centre 

 (mentioned only as a depression in the description), 

 while Savigny's has several systems radiating in narrow 

 ellipses from the centre to the circumference like the 

 plates of a single-starred Madrepore, and without a 

 common orifice. 



If this view be correct the Sidnyum turbinatum of 

 Savigny remains yet to be identified. Mr. Thompson 

 gives two localities for it in Ireland,* but we believe 

 his species to be that of Forbes, with whom he was in 

 constant correspondence. 



Family 6. 



Individuals not much elongated, composed of a dis- 

 tinct thorax and abdomen, and enveloped in a common 

 mass, but without post-abdomen, the heart and 

 ovaries being associated with the digestive organs in 

 the abdomen. 



[This family has been divided into two : Distomidae, 

 containing the genus Distoma ; and Didemnidse, con- 

 taining the genera Didemnum and Leptodinum ; both 

 families also having genera not represented in the 

 British Isles. They chiefly differ in the Distomidse 

 having a colony " rounded and massive, rarely incrust- 

 ing, either sessile or supported upon a long or short 

 peduncle;' and the Didemnidse having a colony 

 " usually flat, thin, and incrusting, rarely thick and 

 massive, never pedunculated." (Herdman.}~\ 



* [Strangford Lough and Belfast Bay, the former on his own authority, 

 the latter on that of Dr. Drummond.] 



