22 Mr. W. H. Dall on certain 
outer corners. Jaw thin, translucent. Gull exactly as in 
Acmea; sides of foot and mantle-edge simple, nearly smooth. 
a 0 ora ; ¢ ° ; 
Dentition >qa_; teeth large, with transverse pectinated or 
denticulate cusps, like those of the large lateral teeth of some 
Tectibranchs or Nudibranchs. 
Pectinodonta arcuata, n. sp. 
Shell white, elongate-ovate, moderately elevated, with a 
blunt polished apex, on which in young specimens remain 
traces of the disk-like chalky embryonic shell; the slopes 
from the apex to the ends both convexly arched, margin 
simple or slightly denticulated by the radiating sculpture ; 
within polished ; scars as in Acmea; epidermis none; sculp- 
ture externally of fine, uniform, rounded, closely-set threads, 
radiating from near the apex to the margin, and reticulated 
by the fine, rather prominent, regular, concentric ridges of 
growth, both ridges and threads averaging near the margin 
about three and a half tothe millimetre. Long. from end to 
end 14°5 millim., from apex to anterior end 5°5 millim. ; lat. 
10°0 millim. ; alt. 5°5 millim. 
Habitat. West Indies; St. Thomas, in coral; Santa Lucia, 
station 215, in 226 fathoms, Blake expedition. 
The examination of a well-preserved specimen showed that 
the end of the muzzle formed a semilunar area with a dis- 
tinctly-marked margin and lappets at the posterior corners. 
In the middle of this flat and nearly smooth area is the mouth, 
surrounded by a small circular papillose area. The jaw is 
thin and translucent, but sufficiently strong to resist contrac- 
tion on the drying up of the soft parts. The radula contains 
about 175 series of teeth, which are large, with strong cusps, 
which are turned toward the middle line of the radula and 
strongly denticulate. ‘The denticulate part, as in most Doco- 
glossa, is nearly black; the anterior denticles are larger, the 
posterior nine subequal in size ; the whole number of denticles 
is twelve; the whole tooth has somewhat the appearance of a 
coarse curry-comb, and suggests that it is due to a consoli- 
dation of the normal three Docoglossal laterals rather than the 
suppression of al] but one and the modification of that one. 
The protrusion of the anterior arch of the head is very 
peculiar and remarkable ; the foot is rather short for the size 
of the shell; otherwise the features are those of Acmea in 
general. The gill is rather large and exactly as in Acmea. 
The number of teeth is the smallest known in any limpet ; 
and none of the same shape have been recorded in the order. 
