ASCIDIANS COLLECTED IN PUGET SOUND. 255 



Ascidiella incrustans, n. sp. PI. XII., figs. 4 — 6. 



External appearance. Body very much depressed, so 

 as to be of flattened scale-like form, and attached by 

 the whole of the under surface. Apertures both on the 

 upper surface, moderately far apart. Surface slightly 

 rough all over. Colour yellowish grey. Length 3' 5 cm., 

 breadth 3" 5 cm., thickness 5 mm. 



Test thin, semi-transparent, highly vascular. 



Mantle thin, musculature fairly strong, chiefly trans- 

 verse. 



Branchial sac plicated. Internal longitudinal bars 

 rather wide, papillated at the angles of the meshes. 

 Transverse vessels alternately large and small, with an 

 occasional much larger one. Meshes almost square, and 

 containing about 7 stigmata each. 



Dorsal lamina a plain membrane with strong trans- 

 verse ribs. 



Tentacles slender, three sizes, closely placed, about 50 

 in all. 



Dorsal tubercle, simple, horseshoe shaped, with the 

 opening between the pointed horns anterior. The nerve 

 ganglion lies close behind the tubercle. The pre-branchial 

 zone is papillated. 



Locality : Off Port Townsend, Puget Sound. 



This species recalls at once the European Ascidia 

 depressa, and some flattened spreading individuals of A. 

 plebeia, but it differs from both these species in several 

 particulars. 



Fig. 4, on PI. XII., shows this species in surface view, 

 natural size ; fig. 5 shows the simple dorsal tubercle in 

 the papillated pre-branchial zone; fig. 6 is a part of the 

 branchial sac from the inside, magnified. 



