from the Gulf of California, etc. 391 
angustato, compresso ; latere postico dilatato; facie dorsali valde declivà, an- 
gulo obtuso limitato, et aream latam, utrinque lineá submarginali partitam, 
ostendente; intus alba, punctis nitidis notata. 
Shell ovate-cuneate, ventricose, thin and fragile, white and 
smooth, covered with a dirty straw-colored epidermis, with here and 
there delicate radiating wrinkles: beaks slightly anterior, tumid 
and decidedly arched ; disk everywhere tumid ; anterior half nar- 
rowed and somewhat acutely rounded at extremity ; posterior end 
broad and distinguished by a ridge running from the beaks to the 
posterior ventral angle, quite acute near the beaks, but becoming 
gradually obsolete ; at this ridge the valves are inflexed nearly at 
right angles, presenting a broad, excavated area, delicately plicated 
near the beaks, and also subdivided by a fainter submarginal ridge. 
At its posterior point the shell is slightly gaping. Lateral teeth 
well developed ; ligament pit large, deep, and oblique ; V-tooth 
small, delicate, supported by bars anteriorly, under which, and 
above the lateral tooth runs a deep excavation. Siphonal sinus 
narrowing to a blunt point. Interior dead white, besprinkled with 
shining dots. 
Inhabits Mazatlan. Lieut. Green. 
The resemblance between this species and M. carinata, from 
the coast of Carolina and the Gulf of Mexico, with which it has 
hitherto been confounded, is very great. Our shell, however, is 
larger, less solid, much more ventricose, and is destitute of the 
crest on the angle which limits the posterior dorsal area, found in 
M. carinata. The anterior end is also narrower, ihe ligament 
cavity smaller and more oblique, and the beaks are more arching. 
ts form is almost precisely that of Unio ovatus. A small African 
Species (M. albida ?) is a miniature of the shell. The posterior 
slope, however, is less vertical, the anterior side less acute; and it 
s deeply impressed concentric grooves in front of the beaks. 
* 
LUTRARIA UNDULATA (PL XV. Fig.7.) 
Lutraria undulata, Gould; Proc. B. S. N. H, Nov., 1851. IV. 89. 
T. candida, PE fragilis, inm Noe ie a cd undata, nisi 
Spicibus vix anticis: latere antic? iiia: iie ‘esti co angustato, com- 
