52 PALAEONTOLOGY OF CALIFORNIA. 



of the posterior above it ; anterior side nearly or quite closed, and 

 the posterior a little gaping. Beaks depressed, approximate, lo 

 cated a little nearer the middle than the anterior margin. Dorsal 

 outline declining obliquely in front of the beaks, nearly straight 

 and horizontal behind them, where the margin is inflected so as 

 to form a narrow false area. Base presenting an irregular, semi- 

 elliptic outline, being rather more prominent behind the middle 

 than in front, where it is even slightly sinuous. Surface orna 

 mented with small, ra'ther irregular, concentric costse or ridges, 

 which diminish in size to mere lines on the umbones. 



Length, about 1.26 inch; height, 0.56 inch; convexity, 0.45 

 inch. 



This species is perhaps most nearly allied to a shell described from the Lower 

 Oolite by Munster, in Goldf. Petref. Germ, ii, p. 257, under the name Lutraria 

 tenuistriata, and subsequently included by Prof. Agassiz in his genus Pleuromya 

 (see Etud. Grit, sur les Mol., tab. 24). It differs, however, from all the forms of 

 that rather variable species of which I have seen figures or specimens, in being 

 more depressed, or longer in proportion to its height, and in having its extremi 

 ties more narrowly rounded ; while its beaks are less prominent and placed farther 

 back. That these differences are certainly not due to accidental distortion is evi 

 dent from the condition of the specimens. Should the name Pleuromya\e sub 

 stituted for Myacites, this shell should be called Pleuromya depressa, since it is a 

 typical Pleuromya. 



