230 DR W. A. HERDMAN ON 



In the branchial sacs of several of these specimens, the vessels forming the 

 apices of the spiral infundibula are considerably swollen, attaining as much as 

 twice their normal calibre (Plate XXXVI. figs. 12 and 13); and the epithelium 

 on the edges of the corresponding stigmata is greatly thickened (see Plate 

 XXXVI. fig. 14). 



Postscript, May 30, 1884. — Since the above paper was written and the 

 plates finished, I have received, through the kindness of Dr P. Herbert 

 Carpenter, three specimens of an interesting and apparently unclescribed 

 Molgulid, which was dredged from a depth of 440 fathoms in the Faeroe 

 channel during the third cruise of the " Porcupine" in 1869. This species will 

 be described and figured in the Report on the " Challenger " Tunicata, Part II. 



EXPLANATION OF THE PLATES. 

 The following system of lettering has been adhered to in all the figures : — 



at. Atrial aperture. 

 br. Branchial aperture. 

 br. f. Fold in the branchial sac. 

 d. I. Dorsal lamina. 

 d. t. Dorsal tubercle. 

 en. Endostyle. 

 (j. Genital organ. 

 h. m. Horizontal membrane of the branchial sac. 

 i. I. Internal longitudinal bar of the branchial sac. 

 m. The mantle. 

 p.,p'. Papillae of the branchial sac. 

 p. p. The peripharyngeal bands. 

 r. The rectum. 



sg. The stigmata of the branchial sac. 

 st. The stomach. 

 tn., tn. The tentacles. 

 tr., tr'., tr". The transverse vessels of the branchial sac. 

 z. The prebranchial zone. 



PLATE XXXV. 



Figs. 1-3. Ascidia plebeia, Alder, var. nov. 

 Figs. 4-6, Polycarpa pusilla, n. sp. 



Fig. 1. Ascidia plebeia, var., seen from right side ; natural size. 



Fig. 2. Ascidia plebeia, var., the test removed, body seen from the left side ; natural size, 



