512 MR. W. HARPER PEASE ON NEW MARINE SHELLS [June 27, 
jugum obsoletis ; mucrone postico, elevato: intus lam. wsert. 
v.-, ant, viii.-x-., post. xii.—xiv.-, centr. unifissatis ; dentibus v. 
post. solidis, declivibus, reliquis acutioribus, omnino pectinatis ; 
subgrundis latioribus, curtis ; sinu latiore, denticulato ; pallia 
squamulis subpilulosis copiose induto. 
Long. °48, lat. °25, alt. ‘07 poll. 
Hab. Society Islands (Pease) (no. 298). 
This curious little species, with the aspect, but not the posterior 
insertion-plates, of Onithochiton, is closely related to confossus, Gld. 
The details of sculpture, however, do not agree ; and the U.S. Expl. 
Exp. species is said to have nine posterior and twenty-five anterior 
insertion-plates, in addition to which the artist has (probably for 
uniformity’s sake) sketched five on each of the central valves! The 
plates are those of Acanthopleura, but the mantle is scarcely bristly, 
the minute hair-like scales being more akin to those of Leptochiton. 
2. Descriptions oF New GENERA AND SPECIES OF MARINE 
SHELLS FROM THE ISLANDS OF THE CENTRAL Paciric. By 
W. Harrer Pease, or Honoutuiv, Corr. Mems. (Com- 
MUNICATED BY P. P. CARPENTER.) 
Genus Lisratuta, Pse. (Family GaLromMip#.) 
T. equivalvis, valvis planis, semilunaribus, medio ligamento junctis ; 
margine cardinali recto, serrato. 
Like Galeomma ; but with the valves flattened as in Placuna, and 
without the ventral gape. They lie wide open. 
LIBRATULA PLANA. 
L. ¢. parva, equilaterali, albida; valvis levibus, marginibus 
tenuibus ; lamina cardinali subprominente, subtilissime serrata; 
umbonibus medianis, parvis, contiguis. 
This singular creature was found living on coral. When at rest, 
the valves are perfectly horizontal, and so closely connected that it 
has the appearance of, and may have been heretofore mistaken for, 
an operculum. The animal does not extend beyond the shell. 
When in motion, however, it protrudes a round lobe to a distance 
as great as its own length, by which it draws itself along, at the 
same time depressing its valves to an angle of about 90°, and raising 
them again when reaching the end of the lobe. This motion of the 
valves is so rapid that, without observing closely, the progress of the 
animal might be supposed to depend on it altogether. It is very 
active in its habits—a rare thing among bivalves—in this respect re- 
sembling Solemya. 
MiITRA SALTATA. 
M. ¢. parva, fusiformi, subulata, tenui, nitida, pellucida, cornea ; 
basi subtruncata, transversim striata, longitudinaliter subti- 
