23 



metricis : intus alba. Cardo valvae dextrae dentibns duobus instructus quorum 

 antico minore, postico bifido ; valvse sinistrae duobus quorum antico bifido, pos- 

 tico perobliquo; dentibus lateralibus nullis; cicatricibus inconspicuis, palliali 

 serie punctorum composito. 



Shell small, rather thin, subglobose, dingy white, marked with 

 delicate lines of growth, which at some parts are more conspicu- 

 ous than at others, and render the surface somewhat irregular ; 

 beaks very nearly median, not prominent ; no distinct lunule in 

 front of them : ligament prominent ; extremities a little above the 

 middle of altitude, very nearly symmetrical. Interior white. Hinge 

 with two direct teeth in the right valve, of which the anterior is 

 smallest, and the posterior is bifid ; and two in the left valve, of 

 which the anterior is bifid and the posterior very oblique ; lateral 

 teeth none ; muscular impressions faint, very large ; pallial impres- 

 sion indistinct, composed of a series of polished dots. 



Length four fifths of an inch ; height six eighths of an inch ; 

 breadth half to five eighths of an inch. 



From San Diego. Lieut. Green. Santa Barbara. Col. Jewell. 



The size and globose form of this species recalls L. columlella, 

 Lk., (L. Adansonii, D'Orb.) : but the shell is much less solid, the 

 beaks less elevated, no areas before and behind them, and the sur- 

 face is not lamellar. L. glolularis may be more like it. Some 

 specimens are nearly perfect spheres, and all of them are very con- 

 vex lenses. 



ARTHEMIS SACCATA (PI. XV. Fig. 2.) 

 Arthemis saccata, Gould; Proc. B. S. N. H., Nov. 1851. IV. 91.) 



T. tennis, albida, ventricosa, rotundato-rhomboidea, subequilateralis, margine 

 ventrali subpendulo; apicibus eminentibus; luuula cordata, linea impressa 

 finita: valvis tumidis, postice compressis et ad marginem sinuatis; cardo valva3 

 dextra dentibus binis fissis instructus, valvoe sinistrre dentibus tribus. 



Shell thin, white, of a rounded rhom.boidal outline and ventricose 

 form, the beaks being prominent, the ventral margin produced or 

 pendulous and the two ends rather suddenly curved above the 

 middle : surface delicately and closely striate, with a zone or two 

 of dusky at stages of growth ; a cordate lunule is defined by an 

 impressed line in front of beaks; the disk of the valves is very 

 tumid, but the posterior umbonal slope is a little constricted, mak- 



