JURASSIC FOSSILS. 51 



anterior side somewhat narrowly rounded; dorsal side sloping 

 and concave in front of the beaks, declining with a slightly con- 

 vex outline posteriorly from the beaks to the middle of the anal 

 margin, which is subangular ; postero-basal margin truncated 

 obliquely forward and downward to a point a little behind the 

 middle of the base; ventral margin rounding down from the 

 front, and extending backwards with a slight descent to its in- 

 tersection with the obliquely truncated postero-basal margin, at 

 which point there is a slight angularity of outline. Each valve 

 provided with an obscure posterior umbonal ridge, which descends 

 with a gentle curve from behind the beak to the postero-basal 

 angle. Surface with moderately distinct, irregular, concentric 

 striae. 



Length, 1.62 iuch; height, 1.45 inch; convexity, 1.10 inch. 



As we know nothing of the hinge, or muscular and pallial impressions of this 

 shell, its generic relations cannot be determined beyond doubt. From its thinness, 

 however, and general appearance, we have been led to refer it provisionally to 

 D'Orbigny's genus Unicardium. Its most marked characters are its posterior 

 umbonal ridges, and truncated postero-basal margins. We are not acquainted 

 with any closely allied species. 



FAMILY ANATINID^l. 



Genus MYACITES (Schlot.), Minister, 1840. 



Mtacites depressus, Meek. 



PI. 8, Fig. 6, and 6 a. 



Shell rather small, depressed, narrOw-subelliptic, moderately 

 gibbous in the middle, and along the flanks obliquely backwards 

 and downwards from the umbones ; slightly compressed in the 

 antero-ventral region. Extremities very narrowly rounded, — the 

 most prominent part of the anterior being below the middle, and 



