18 



angustato, compresso ; latere postico dilatato ; facie dorsali valde declivii, an- 

 gulo obtuso limitato, et aream latam, utrinque linci submarginali partitam, 

 ostcndente ; 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, arid 

 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, the ligament 

 cavity smaller and more oblique, and the beaks are more arching. 

 Its form is almost precisely that of Unio ovatus. A small African 

 species (M. alUda ?) is a miniature of the shell. The posterior 

 slope, however, is less vertical, the anterior side less acute, and it 

 has deeply impressed concentric grooves in front of the beaks, 



LUTRAKIA UNDULATA (PI. XV. Fig. 7.) 

 Lutraria undulata, Gould ; Proc. B. S. N. H., Nov., 1851. IV. 89. 



T. Candida, papyracea, fragilis, ovata, postice hians, concentrice unilata, nisi 

 prope marginem posticum et ad aream impressam dorsalem concinne rugosa ; 

 apicibus vix anticis : latere antice rotundato ; latere postico angustato, com- 



