106 
THE VOYAGE OF H.M.S. CHALLENGER. 
straightened it measured 4 '3 cm. The rhinophoria were deeply retracted but existed ; 
the branchia and a large portion of the branchial cleft were missing, and all the alimentary 
tract as far as the buccal tube. Another animal had very likely entered through the 
branchial cleft, and had eviscerated its host. 
Thor disa, Bergh. 
Thordisa, Bergh, Malacolog. Untersuch. (in Semper, Reisen im Archip. d. Philipp. Th. II. 
Bd. ii.), Heft xii., 1877, pp. 540-542. 
Forma corporis fere ut in Discodoridibus ; tentacula tuberculiformia ; branchia pauci- 
foliata. 
Armatura labialis nulla. Lingua rhachide nuda, pleuris multidentatis ; dentes 
hamati. — Penis inermis. 
This genus, to which the species to be presently described can be only doubtfully 
assigned, is as yet but little known. In the general form of the body it comes near 
Discodoris, but diff ers in the entire absence of labial plates. The armature of the tongue, 
however, is similar ; the rhachis is naked and provided with numerous lateral teeth, of the 
usual hooked form. The penis is unarmed. 
The three following species which the genus contains are but little known ; they are 
all tropical forms, but nothing is known of their mode of life. 
1. Thordisa maculigera, Bergh. 
Philippine Sea. 
2. Thordisa villosa (Alder and Hancock). 
Indian Ocean. 
3. Thordisa ? clandestina, n. sp. 
Pacific. 
Thordisa clandestina, n. sp. (PI. III. figs. 21-25). 
Habitat. — Western Pacific (Torres Strait). 
A single specimen was taken on September 8, 1874, at Torres Strait (Station 186, 
lat. 10° 30' S., long. 142° 18' E. ; depth, 8 fathoms ; bottom, coral sand). 
The specimen was well preserved in alcohol. 
Its length was 18 mm., breadth 9 mm., and height 6 mm.; the breadth of the 
mantle edge 3 '5 mm., of the foot 6 ‘5 mm.; the length of the tail 175 mm., the height 
of the extended rhinophoria 3 mm., of the (outstretched) branchia 2 '5 mm. The colour 
was whitish ; on the back were a number of brownish-black spots, which also were present 
and more abundant on the sides of the body and under-surface of the mantle edge; the 
