1917.] NATURAL SCIENCES OF PHILADELPHIA. 39 



The convexity of the ribs is, in fact, barely perceptible, though they 

 are perfectly obvious to the eye. Where eroded quite superficially, 

 the ribs are reduced to flat spaces between slightly raised radii of 

 half their width, representing the intervals. With further erosion 

 the ribs are transformed into slightly concave radii with the edges a 

 little raised, and separated by very narrow, deeply bitten crevices 

 occupying the middle of the original intercostal valleys. With 

 further etching, the edges of the original ribs become more prominent, 

 the middle being eaten out, so that the effect is an even radial striation. 

 The basal margin is strongly toothed within. The cardinal area is 

 moderately wide. Teeth not examined. Height 44.4, diam. 32 mm.; 

 length about 42 mm. 



This species resembles G. gigantea Reeve of the west coast, and also 

 G. jamaicensis Dall, of the Bowden bed. It differs from both in 

 wanting very minute radial striae. 

 Glycymeris trilobioosta n. sp. PI. 6, fig. 5. 



The shell is compressed, equilateral, with small beaks; rather 

 thick. Sculpture of about 32 radial ribs, the small ones at the ends 

 simple, the rest brace-shaped ('— — ^) , their interstices narrow grooves ; 

 towards the beaks the cross-section becomes simpler, the ribs at 

 first being convex. Over the ribs there is a very regular, close 

 sculpture of minute concentric threads. The basal margin is toothed 

 within, as usual. Cardinal area small, having about 3 V-shaped 

 grooves. Teeth 12 on each side, rather long, smooth-sided, those of 

 the median part a little bent angularly. 



Height 14.6, length 14.5, semidiameter 4 mm. 



The peculiar form of the ribs and the beautiful concentric sculpture 

 over them are the chief characters of this species. 

 Glycymeris lloydsmithi n. sp. PI. 6, fig. 6. 



The shell is moderately convex, slightly inequilateral, the margins 

 somewhat straightened on both sides of the umbo. Sculpture of 

 about 27 very low, broad, rounded ribs, rapidly . becoming weaker 

 towards the base, faint on the anterior end, which is bisected by an 

 indistinct radial angulation, as in G. jamaicensis and some recent 

 Antillean species. No secondary radial striation can be seen. The 

 cardinal area is very small, not grooved. There are 11 teeth before 

 and 12 behind the beaks; most of the posterior teeth and a few of the 

 anterior being angularly bent. The articulating faces of all of the 

 teeth have very minute striation vertical to the tooth. The lower 

 half of the inner margin of the shell shows about 20 prominences of 

 the usual form. 



