336 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
flattened, the largest one being only 16 mm. high. Upper surface covered by 
firm plates, evidently imbricated in the very small specimens, but not in the 
large ones. In the smallest specimen there are about three hundred plates, but 
more than two thirds of these are very small plates arranged in about three rows 
around the margin; of the remainder, 20 are included in the oral area and 10 in 
the anal. In the largest specimens the number of plates is considerably greater, 
there being 30 plates in the oral, and 20 in the anal area. But several features 
are common to all the specimens, regardless of size. The plates are so finely 
granulated that they feel smooth and appear so to the naked eye. The anus is 
closed by 5 or 6 large plates and several small ones, while the mouth is guarded 
by 5 broad triangular plates, alternating with 5 long narrow ones which lie be¬ 
neath them. Between the oral and anal areas there are about four transverse rows 
of large plates. None of the plates, either on the margin or elsewhere, are 
pitted, or perforated for the passage of either pedicels or papillae. Ventral sur¬ 
face almost flat, depressed below edge of shell. Pedicels arranged around mar¬ 
gin in three or four rows (in very small specimens two rows), and with a fairly 
well-defined double row down the middle of the disc. This middle series is 
wanting or is very indistinct in the smallest specimens. There is also a single 
row of smaller or imperfect pedicels close to the margin of the disc, on the under 
side of the marginal plates of the dorsal surface. Retractor muscles strong. 
Intestine long and much coiled. Respiratory trees very unequal, the right one 
much longer. Cuvier’s organs wanting. Reproductive gland, of numerous, long, 
unbranched filaments. Polian vessel one, short and globular. Stone-canal 
one, on left side, somewhat pear-shaped, more or less attached to mesentery. 
Calcareous ring (fig. 6) without posterior prolongations, the radial and interradial 
pieces of approximately equal size. Tentacles 10, plentifully filled with support¬ 
ing rods (fig. 7) even to the tips of the branches, where they are very small (fig. 8). 
Pedicels with terminal plates and long, narrow, knobbed, perforated plates (fig. 9) 
well filling the walls. The skin of the ventral surface is thin and somewhat 
translucent, but is thickly crowded with calcareous plates (fig. 10) which are 
easily seen with a small magnifying glass. These plates are generally more or 
less elliptical, very much perforated with minute holes, and with numerous, low, 
rounded knobs all over them. Others have an irregular and low spire, of no 
very definite shape or position. These plates seem to be confined almost wholly 
to the ventral surface. Color very pale pinkish white; tentacles bright flesh-red. 
The latter are much contracted, and their appearance gives the impression that 
the color of the rest of the animal has faded in the alcohol. 
• N 
There are no data with these specimens, but I believe they were 
collected at Neah Bay, as Mr. Harrington speaks of collecting a 
Psolus at that point. They are all much contracted, and in their 
contracted condition they look not a little like chitons, and for that 
reason, the name selected has been given them. This species re¬ 
sembles P. operculatus of the Caribbean Sea in some particulars, 
but differs from it quite markedly in the absence of granulation, the 
arrangement of the feet, and the deposits in the sole. It is the first 
