GYMNOTOPLAX. 211 
two smaller and narrower posterior tentacles, which are flattened 
and folded; on the left side there is a conspicuous dark blue eye 
behind the base of the dorsal tentacle, but on the right side the eye 
is concealed or wanting in our specimen. ‘The gill occupies the 
groove below the mantle on the right side, and is nearly one-third 
the length of the shell; just in front of the gill there is a low 
rounded prominence with a central orifice. The shell is thin, trans- 
lucent, pale yellowish-white, oblong, with the sides nearly parallel 
and the anterior end bluntly rounded. The spire is a little promi- 
nent, terminal and strongly curved to the left, with the nucleus 
smooth, glassy and incurved, situated at some distance from the 
margin. The surface of the shell is covered with numerous strong, 
irregular, concentric undulations, and by much smaller and finer 
lines of growth, which are crossed by microscopic, interrupted, 
radiating lines, giving a very finely reticulated appearance. Length 
of the shell 13:5, breadth 8°5 mill. ( Verril/). 
Off Martha's Vineyard, 250 fms. (U.S. Fish Comm.). 
Pleurobranchus americanus V., Trans. Conn. Acad., vi, p. 429, 
pl. 44, f. 13 (April, 1885). 
G. MARTENS! Pilsbry, n.n. PI. 48, figs. 34, 35. 
Mantle deeply incised in front, projecting beyond the foot all 
around, the margin thickened and somewhat ragged, evenly and 
finely granose all over, yellowish-brown, with round spots of dark 
purple-brown, much scattered, more numerous near the periphery 
and at the circumference of the part of the mantle covering the shell. 
Tentacles and angles of the head dark purple-brown. Gill extend- 
ing along nearly the entire hind half of the right side. Length of 
living animal 18 mill.; of alcoholic specimen 10; breadth 72 mill., 
the foot 6 long, 32 wide; gill 24 long. 
Shell comparatively very large, elongated, oval, flat, thin and 
pale reddish, with the apex curved toward the right, and growth- 
striz strong. Length of shell 6, breadth 23 mill. (Mart.). 
Fouquets, Mauritius (Mobius). 
Pleurobranchus scutatus Mrs., in Mobius’ Meeresfauna Mauritius, 
p. 309, pl. 21, f. 8 (1880). Not P. seutatus Forbes, 1844. 
This apparently new species resembles Berthella porosa Blainv. 
(see Gray, Figs. Moll. Anim., pl. 43, f. 1) in aspect, but is distin- 
guished by the deep anterior cleft in the mantle. It is remarkable 
that the smaller species of Plewrobranchus have, as a rule, relatively 
