566 GEOLOGY OF OHIO. 



Plectambonitcs transversaiis , Wahlenberg. 



(Plate 25, Fig. 5, a, b ; Plate 30. Fig 13; Plate 31, Fig. 6.) 



The variety proloiigata occurs at Soldiers' Home, Fauver's Quarry, 

 and Huffman's Quarry, Ohio, in the upper shaly courses. The hinge 

 area is long and narrow, its inner anterior border is straight. The car- 

 dinal process is elongate triangular. The two divergent crural plates are 

 closely approached to the process, and have coalesced with it below, so 

 that the cardinal process falsely appears to be trilobate. No interiors 

 have been found. The great width of the shell is the distinctive feature. 

 It is usually strongly bent near the beak, and is much more moderately 

 convex anteriorly. (Plate 25, Figs. 5 a, b; Plate 30, Fig. 13.) 



The variety elegantula y including the forms with medium width of 

 shell, rather more equal convexity antero-posteriorly, and with some of 

 the striae rather decidedly elevated above the general surface of the 

 shell, is common at Hanover, Indiana. (Plate 31, Fig. 6.) 



Leptaena rhomboidalis, Wilkins. 



During the earlier stages of growth the pedicle valve is moderately 

 convex and the brachial valve still more moderately concave. In some in- 

 dividuals the curvature of the shell remains moderate until the geniculation 

 is reached, the latter being found at the last anterior and usually strongly 

 elevated concentric wrinkle; shells of this description are of medium 

 size, and occur at Brown's Quarry, Soldiers' Home, and elsewhere. In 

 other individuals the curvature of the shell posterior to the geniculation 

 is more marked, the last concentric wrinkle is less sharpby elevated, and 

 the anterior part of the shell beyond the geniculation is far more devel- 

 oped, almost equaling at times the part posterior to the geniculation in 

 length; shells of this kind are of largest size, and occur at Soldiers' 

 Home, Huffman's Quarry and elsewhere. In addition to the concentric 

 wrinkles occurring posterior to the geniculation, numerous fine radiating 

 striae exist, crossed by often still closer, less prominent concentric striae. 

 Hinge area of the pedicle valve very narrow but still broader than that of 

 the opposite valve. The delthyrium is closed by a convex triangular del- 

 tidium except at its apex where one of our specimens shows a small 

 foraminal opening. The teeth are quite short and very oblique, a groove 

 defining their postero-lateral side. 



The muscular area is strongly bordered laterally by a very narrow but 

 sharp ridge. In a small valve the sides slope outward a short distance 

 anterior to the hinge area, and then inwards with a email curve which 

 increases anteriorly, where the border almost disappears. The general 

 outline of the muscular area in this case was obovate. In a mature 

 strong!) 7 curved pedicle valve the postero-lateral borders of this area 



