180 DISCOVERY REPORTS 



Family APLUSTRIDAE 



Genus Parvaplustrum n.g. 



Type: Parvaplustrum tenerum n.sp. 



A minute, white, extremely fragile, Haminea-like shell occurred abundantly at depths between 

 ioo and 320 m. in fourteen dredgings, mostly north and north-west of the Falklands. 



It is a tectibranch of uncertain affinity, but probably lies nearest to Aplustridae. There are four 

 cephalic tentacles, the outer two with a dorsal groove, rendering them almost auriculate, the inner two 

 simple, no apparent eyes, a prominent bilobed proboscis and two protective shields to the head, the 

 upper one formed by the mantle and its lower counterpart by a widening of the foot, which posteriorly 

 tapers to a broadly V-shaped tail. The stomach plates are numerous, small, cartilaginous, more or less 

 diamond-shaped bodies, which are closely grouped like paving stones (Fig. M, 98). The radula is 

 a rake-like device composed of long, slender, arcuate rods (laterals), each with a shorter and even more 

 slender forked member set directly underneath it (Fig. M, 96). The form of the gills could not be 

 determined owing to the unsatisfactory preservation of the internal structures. The shell is lightly held 

 along the outer edge of the aperture by a thin reflexion of the mantle, and even when the animal is 

 fully expanded most of the shell is probably exposed. The animal is apparently completely retractive 

 (Fig. M, 97, p. 195). 



The presence of four cephalic tentacles suggests relationship with either Aplustrum or Hydatina, the 

 only other tectibranchs, apart from Pterygophysis and the Aplysiomorpha, possessing two pairs of 

 cephalic tentacles. The Aplysiomorpha have neither head disk nor dorsal shield and Pteryophysis has four 

 conspicuous wing-like pleuropodial lobes. The stomach-plates resemble those of Akera bullata Mueller 

 (Tryon & Pilsbry, 1893) but the radula is not comparable with that of any tectibranch known to me. 



Not only a new genus but probably a new family is represented by this anomalous species, but 

 I hesitate to erect a new family without recourse to better preserved material. 



Parvaplustrum tenerum n.sp., PI. VII, fig. 25 



Shell minute, pure white, thin and extremely fragile. Ovate-globose. Body-whorl occupying full 

 height of the shell. Aperture wide below, but somewhat constricted above. Spire a shallow cavity 

 occupied by a low, convex, smooth protoconch of one translucent whorl. Outer lip thin, retracted 

 to form a moderately deep sinus with a narrowly rounded termination; basal lip broadly arcuate. 

 Sculpture consisting of extremely fine and dense spiral striations. 



Height 2-8 mm.; diameter 2-15 mm. (shell of holotype, St. WS 219). 



Type locality. St. WS 219. North-west of Falkland Is., 47 06' S, 62 12' W, 3 June 1928, 



116-114 m. 



St. 51. Off Eddystone Rock, East Falkland Is., from 7 miles N 50° E to 7-6 miles N 63 E of Eddystone Rock, 



4 May 1926, 105-115 m. 

 St. WS 210. North of Falkland Is., 50 17' S, 6o° 06' W, 29 May 1928, 161 m. 

 St. WS 211. North of Falkland Is., 50 17' S, 6o°o6' W, 29 May 1928, 161-174 m. 

 St. WS 213. North of Falkland Is., 49° 22' S, 6o° 10' W, 30 May 1928, 249-239 m. 

 St. WS 214. North of Falkland Is., 48 25' S, 6o° 40' W, 31 May 1928, 208-219 m. 

 St. WS 216. North of Falkland Is., 47 37' S, 6o° 50' W, 1 June 1928, 219-133 m. 

 St. WS 220. North-west of Falkland Is., 47 56' S, 62 38' W, 3 June 1928, 108-104 m - 

 St. WS 227. North-east of Falkland Is., 51 08' S, 56 50' W, 12 June 1928, 320-295 m. 

 St. WS 229. North-east of Falkland Is., 50° 35' S, 57 20' W, 1 July 1928, 210-271 m. 

 St. WS 234. North of Falkland Is., 48 52' S, 6o° 25' W, 5 July 1928, 195-207 m. 

 St. WS 235. North of Falkland Is., 47 56' S, 6i° 10' W, 6 July 1928, 155-155 m. 



