184 PALAEONTOLOGY OF CALIFORNIA. 



the posterior side; base broadly and pretty regularly convex, 

 joining the other two sides of the triangle by narrowly rounded 

 angles. Internal basal margin minutely dentate, the dentations 

 corresponding with the external ribs. Surface ornamented by 

 numerous fine, regular, radiating ribs, flattened on top, and with 

 the interspaces somewhat narrower than the ribs. 



Length, 1.1 inch ; width, .8 inch ; height of single valve, .18 inch. 



Eare in the Tejon Group, ten miles west of Griswold's, southeast of the Sheep 

 Well, on the road from San Juan to New Idria. 



This shell is remarkable for its thinness, and as much of the hinge as I have been 

 able to expose, shares the delicate character of the shell itself. 



VENUS, Linn. 



V. JEQUILATERALIS, n. S. 

 PL 30, Fig. 76. 



Shell small, triangular, the three sides forming a nearly equi- 

 lateral triangle, with curved sides; beaks small, pointed forwards, 

 and placed very slightly in advance of the middle; cardinal mar- 

 gin sloping rapidly and slightly convex; base broadly and not 

 very prominently convex, rounding up in advance, and uniting 

 by a short curve with the anterior end; the posterior basal angle 

 is much more acute than the anterior ; anterior margin slightly 

 excavated under the beaks, nearly straight below. Surface marked 

 by small concentric ribs. Hinge composed of short, robust teeth; 

 muscular scars large, the anterior being the deepest and most 

 strongly marked ; pallial sinus shallow ; internal margin plain. 



Length, .85 inch ; width, .75 inch ; height of single valve, .2 inch. 



From the Tejon Group, from San Diego. 



The nearest ally of this species is V. lenticularis, G., from Benicia, but the two 

 shells can be easily distinguished by the more marked triangular form, and smaller 

 size of the present one, and by the absence of a lunule, or, if it does exist, by its 

 being large and very faintly marked. In V- lenticularis the lunule is small but 

 distinct. 



