274 PALAEONTOLOGY OF CALIFOKNIA. 



lobe of Pitcherii. Base entire ; sides sloping from the hinge line 

 to the base at nearly equal angles, the right side a little convex, 

 the left about equally concave. Surface more or less squamose, 

 sometimes a series of very rough imbrications on the umboual 

 ridge. Hinge long, narrow, and curved laterally with the de- 

 flection of the beak. Upper valve small, thick, narrow, flat, and 

 oblique. 



For the resemblances and differences between the three forms, see notes following 

 the description of the succeeding species. 



G. mucronata, Gabb. 



(G. Pitcherii, Eoem. {not Morton) ; Kreid. Tex., p. 73, pi. 9, fig. 1.) 



(G. Pitcherii, Schiel? {not Morton) ; Pacific Kailroad Keport, Vol. 2, p. 108, pi. 3, 



fig. 9.) 



Shell long, narrow, subtrigonal, very thick. Lower valve 

 strongly bicarinate by two radiating ridges, one running from 

 the umbones, the other near the opposite margin, dividing the 

 surface rudely into three portions, two marginal planes and an 

 intermediate one, often concave. Beaks very prominent, pro- 

 duced, median and strongly incurved; umbones very prominent 

 and rounded. Hinge long, narrow, and straight. Sides nearly 

 equal, diverging equally at a narrow angle; base slightly convex, 

 entire. Upper valve long, narrow, thick, and straight. Surface 

 marked by subsquamose lines of growth. 



The differences between these three species are as follows : G. Pitcherii is dilated 

 and very distinctly bilobate, the beaks are small and incurved, and the umbones 

 are broadly rounded. G. navia is narrow triangular, the beaks are deflected ex- 

 cessively, and the umbonal ridge is acute-angular, sometimes not being more than 

 forty to forty-five degrees. The hinge is also deflected. G. mucronata is also 

 narrow, but the beaks, which are very long and incurved, in some cases into a 

 prominent hook, are placed exactly in the median line, and the umbone and um- 

 bonal ridges are rounded, never even subangular. 



Mucronata and Pitcherii agree in the median beaks, but differ in form. Navia 

 and mucronata have the narrow trigonal form, but differ in the size, shape, and 

 positions of the beaks. There is less resemblance between these latter on the one 

 side, and Pitcherii on the other, than there is between some of the varieties of 



