Mr. J. Alder on the British Tunicata. 171 



other respects leaves little doubt of its identity with his species. 

 The size of the mass is always variable, depending upon the age 

 and other circumstances. Amarcecium pomum comes very near 

 to the A. Nordmanni of Milne-Edwards; but it differs in colour, 

 in the size of the individuals, and in the greater length of the 

 postabdomen. 



Amarcecium papillosum, n. sp. 



Common body depressed, sessile, yellowish fawn-coloured. In- 

 dividuals prominent, rising into distinct papillae over the surface, 

 and forming numerous, irregular, close-set, ill-defined systems, 

 set round wide common orifices. Branchial aperture with six 

 obtuse lobes. Thorax brownish fawn-coloured. Abdomen rather 

 darker : postabdomen longish, cylindrical. Diameter of mass 

 about an inch ; height about one-third as much. 



Two specimens of this Amarcecium were obtained by dredging 

 in shallow water, Menai Straits, in 1852. 



Sidmjum turbinatum, Savigny. 



A compound Ascidian sent by Dr. Leach to Savigny, from the 

 English coast, is described under this name by the latter, in a 

 short Appendix to his Memoire. Two different species have 

 been referred to it by British authors, but, I think, erroneously. 

 Dr. Fleming found what he was "inclined to consider" the S. 

 turbinatum on the rocks of the Isle of May. His description in 

 1 British Animals ' is compounded of the characters of Savigny's 

 genus combined with those of his own recent specimen. What 

 we consider to be Dr. Fleming's species is not uncommon on 

 the eastern coast. Prof. Edward Forbes, again, has described 

 another species as the Sidnyum turbinatum of Savigny, and has 

 altered the generic character to suit it. It is only necessary, 

 however, to pay a little attention to Savigny's description to be 

 convinced that our distinguished English naturalist was under 

 a mistake, and that his species, which is composed of short, 

 cylindrical, truncated masses, nearly as broad below as above, 

 the individuals of which have a branchial aperture of eight rays 

 and a broad postabdomen (see Brit. Moll. pi. b. f. 2), cannot be 

 the animal described by Savigny with a turbinated common 

 body, contracted below, the individuals having a branchial aper- 

 ture with six rays, and a pedunculated postabdomen, dilated 

 and filiform {mince comme un fil) ; besides which, Forbes's spe- 

 cies has a common excretory orifice (mentioned only as a depres- 

 sion in the description), removing it to a different section from 

 Savigny's Sidnyum, which, like Aplidium, is without this cha- 

 racter. Add to which, the individuals of Sidnyum are arranged 

 in narrow ellipses radiating from the centre to the circumference, 



12* 



