288 DB. W. Ai HEBDMAIT ON BRITISH TUNTCATA. 



coating of sand-grains, and in others by large pieces of shell, 

 nullipore, and stones. Heller's figure is fairly characteristic ; in 

 many of my specimens, however, the atrial siphon is directed 

 more posteriorly than dorsally, and is not so prominent. 



I figure a specimen with the test removed (PI. XIX. fig. 1), 

 to show the elongated atrial siphon. 



The characters of the internal organs are as follows :— 



Branchial sac oblong, not longitudinally plicated, or only very 

 slightly so. Transverse vessels stout, all the same size. Internal 

 longitudinal bars conspicuous, bearing large papillae at the angles 

 of the meshes, and smaller, more conical intermediate ones ; these 

 latter are absent in a few of the meshes. Meshes slightly longer 

 vertically than transversely, each containing three to five rather 

 short stigmata. 



Dorsal lamina transversely ribbed, margin minutely denti- 

 culated. 



Tentacles inconspicuous, very slender, 72 in number, of three 

 sizes arranged alternately — 18 large, 18 medium-sized, and 36 

 small. 



Olfactory tubercle oblong in outline, the opening at the ante- 

 rior end of the right side ; horns almost touching, not coiled. 



This species comes near Ascidia depressa, Alder. 



The condition of the olfactory tubercle described above (and 

 figured on PL XVIII. fig. 7) is the one usually found j but in 

 some specimens the horns are turned in (as shown on PI. XVIII. 

 fig. 6) but not coiled. In some parts of the dorsal lamina (see 

 PI. XIX. fig. 3) a series of papilla? is present, forming a line 

 parallel to and resembling the denticulated margin. 



Ascidia exigua, n. sp. (Plate XIX. figs. 5-8.) 



External appearance. — Shape ovate-elliptical, anterior end 

 slightly wider than posterior, both blunt and rounded; depressed, 

 flattened laterally. Attached by the entire left side ; margin 

 slightly expanded here and there. Apertures both sessile, incon- 

 spicuous ; branchial terminal, atrial about two fifths of the way 

 down. Surface smooth. Colour yellowish grey. Length 1*1 

 centim., breadth 06 centim. 



Test moderately thick, strong, transparent } no vessels visible. 



Mantle well developed ; muscular bands very delicate, but close, 

 forming rather a fine network. Ocelli red, minute. 



Branchial sac not longitudinally plicated, meshes having the 



