JURASSIC SPECIES. 131 



PINNIDJi]. 



Genus PINNA, Linnaeus. 

 Pinna Kingii, Meek. 



Plate 12, figs. 9, 9 a. 



Shell attaining about a medium size, very thin, elongate-subtrigonal in 

 general outline, apparently slightly arcuate, rather convex, with a more or 

 less defined angle extending along the middle of each valve, so as to give the 

 transverse section, especially at almost any point between the middle and the 

 beaks, a rhombic subquadrangiilar outline, becoming more compressed pos- 

 teriorly; upper and lower margins converging rather rapidly to the beaks, 

 the first being very nearly straight, and the latter slightly convex; posterior 

 extremity a little obliquely truncated, but rounding into the base below; 

 valves flattened from the mesial angle to the upper and lower margins. Sur- 

 face marked by fine lines of growth running parallel to the posterior and lower 

 margins, and showing a tendency to gather into little wrinkles, particularly 

 near the latter; while above the lateral angle they are crossed by about ten 

 to twelve obscm'8 radiating costse, most distinctly defined near the beaks, and 

 gradually becoming obsolete near the middle. 



Apparently attaining a length of about 4 inches, with a height at the 

 posterior end of about 1.90 inches; convexity, 0.85 inch. 



The only specimen of this species contained in the collection is in an 

 imperfect condition, though sufficiently well preserved to show its form and 

 surface-markings, as well as its remarkable thinness. In form and size, it 

 seems to have been much like P. canceUata of Bean (Morris and Lycett's 

 Monogr. Moll. Great Oolite, tab. xiii, fig. 20 a h), from which it differs in 

 having a mesial angle along each valve, and in having its radiating costse 

 obsolete on the posterior part of the valves. These costa3 likewise differ 

 in being merely obscure ridges on our shell, about as wide as the furrows 

 between, instead of mere "knotted lines". It is, however, about as nearly 

 allied to some imperfectly-known Carboniferous species of the Mississippi 

 Valley. 



Locality and position. — Weber Canon, Wasatch Range; possibly Ju- 

 rassic ; though it may be Carboniferous, as it was found loose, and rocks be- 



