DE. W. A. HEEDMAN ON BEITISH TTJNICATA. 287 



This species has such a characteristic appearance that no ad- 

 dition to Alder's description is necessary. No account of its 

 internal characters, however, has been given — Alder merely 

 saying " Branchial sac finely reticulated, with tubercules at the 

 intersections," and Heller giving no information beyond the 

 external appearance. This being the case, the following notes on 

 the condition of the branchial sac &c. are necessary to complete 

 the description of the species. 



Branchial sac long and narrow, generally extending beyond 

 the viscera posteriorly, longitudinally plicated ; transverse vessels 

 all one size. Meshes square or slightly elongated transversely, 

 with short blunt papillae at the corners, and smaller intermediate 

 ones on the internal longitudinal bars. Stigmata elongate-ellip- 

 tical, rather short, four or five in a mesh. 



Dorsal lamina strong, not very wide, transversely ribbed ; 

 margin provided with short teeth opposite the ends of the ribs 

 and generally one or tw r o smaller intermediate ones. 



Tentacles simple, tapering, and slightly curved, numerous and 

 closely placed, about 15 to 20 longer and stouter, between each 

 pair of which are one or two small ones. 



Olfactory tulercle simple, oval in outline, with the opening at 

 the anterior slightly narrower end. 



Viscera in a compact mass, occupying the posterior half of the 

 left side of the branchial sac. 



Locality. Attached to the under surfaces of large stones at 

 extreme low water, near King's Cross Point, Lamlash Bay. 



Ascidia plebeia, Alder. (Plate XVIII. figs. 6-8 ; Plate XIX. 

 figs. 1-4.) 



Ascidia plebeia, Alder, Ann. §• Mag. Nat. Hist. ser. 3, vol. xi. p. 155 

 (1863); Alder, Hebrid, Invert, Brit. Assoc. Rep. for 1866, p. 207 

 (3867); Heller, Untersuch. ii. d. Tun. Adriat. u. Mittelm. 1 Abtli. 

 p. 14, Taf. v. fig. 7 (1874). 



This is a rather variable species, showing very different appear- 

 ances according to its position and the objects to which it is 

 attached; it also varies greatly in the amount to which it is 

 covered by sand, shells, and other foreign bodies. Alder says 

 " slightly covered with fragments of shell and sand at the attached 

 end." Some of my specimens from Lamlash Bay have the soft 

 dull-green test entirely exposed, while others are completely 

 covered, even on the siphons in some cases, by a fine, but thick 



