64 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4thSer 



concentric lines of growth which disappear on the earher por- 

 tions of the shell ; a prominent ridge or angulation extending 

 from the beaks diagonally to the posterior extremity ; hinge 

 typical of this genus ; muscular impressions inaccessible. 



Dimensions: — Altitude, 9 to 10 mm.; length, 10 to 13 mm.; 

 thickness, 6 to 7 mm. 



Occurrence : — Found abundantly in the lower Miocene of 

 Kern River, California, locality 69. 



Though much smaller in size, the shells of this species are 

 almost exact prototypes of Spisula exoleta Gray as figured and 

 described by Arnold, and as represented by a sample collected at 

 San Diego. More than one hundred specimens of M. sectoris 

 were obtained from the locality given above, and they were all 

 nearly equal in size, there being no gradation between them and 

 larger specimens. This fact seems sufficient to warrant its de- 

 scription as a new species. 



Type: — No. 131, and cotype No. 132, Cal. Acad. Sci., on the 

 south and west slopes of Pyramid Hills, about 15 miles north- 

 east of Bakersfield. 



GASTROPODA 

 Genus CALLIOSTOMA Swains. 

 Callistoma pacificum, new species 

 Plate 8, figures 2a and 2b. 



Shell conical, thick, with about five convex whorls ; spire 

 moderately high; whorls of the spire ornamented with ten 

 almost equally prominent spiral threads separated by narrower 

 interspaces; two of the spiral threads nearest the posterior 

 margin are centrally grooved ; suture distinct, impressed ; body- 

 whorl slightly concave near the suture, convex over the central 

 portion, sharply rounded at the base, sculptured the same as the 

 whorls of the spire; the base ornamented with eighteen or 

 twenty very fine spiral lines ; aperture subcircular ; outer lip 

 smooth ; columella thickened and incrusted. 



Dimensions : — Altitude of the type specimen, 15 mm. ; diam- 

 eter of the last whorl, 14.5 mm. 



