CRETACEOUS FOSSILS. 170 



A. Mathbwsonii, n. s. 



PI. 30, Fig. 258. 



Shell small, compressed, subquadrate, length and breadth 

 about equal ; beaks very prominent, strongly incurved, presented 

 forwards, and overhanging the anterior end ; cardinal margin 

 straight and sloping downwards; posterior truncated; anterior 

 deeply emarginate above, narrowly and prominently rounded 

 below. Surface marked by fine, irregular, concentric lines, va- 

 riable in different specimens, but usually most prominent near 

 the beaks. Lunule deeply impressed. 



Figure, natural size. 



Locality: Near Martinez. All the specimens I have seen were collected by Mr. 

 Mathewson. 



A. Tuscana, n. S. 



PI. 30, Fig. 257. 



Shell thick, narrow, cuneate; beaks small, incurved; cardinal 

 margin straight, sloping downwards to the posterior end, which 

 is obliquely and convexly truncated ; anterior end straight and 

 sloping outwards above, rounded below; base most prominent 

 below the beaks, nearly straight behind, rounded upwards in 

 advance. Lunule small, deeply sunken. Surface marked with 

 regular, rounded, concentric ribs. 



Figure, natural size. 



Localities: From Division A., at Tuscan Springs; also at Pence's Eanch, Butte 

 County. 



Allied to A. Conradiana ; but narrower posteriorly, more sloping on the cardinal 

 border ; the beak is also less terminal, and the anterior end is of an entirely dif- 

 ferent shape. Two specimens in the collection of the Survey, from Chico Creek, 

 resemble this species in most of the essential characters, except that they are twice 

 as large, are more robust, wider from beak to base, and slightly excavated on the 

 anterior end, immediately under the beaks. They are also proportionally nar- 

 rower posteriorly than the present species, and may prove to be distinct ; but, 

 without more material, I would not venture to name them. 



