254 



Ventral valve below medium size, moderately convex, transverse, Avitli the 

 proportions of length to breadth about as 3 to 4 ; hinge line equal to the great- 

 est width of test ; cardinal extremities apparently acute-angular. The valve 

 is most elevated just posterior to the middle, whence to the front the surface 

 extends in a moderate slope, and is slightly curved, but towards the beak it is 

 more strongly curved. The valve curves regularly and moderately strongly 

 across the middle, becoming very slightly flattened towards the sides. The 

 cardinal angles are also somewhat flattened. The inner mould of the valve is 

 marked by small, subangular, radiating, raised lines, which, on the single 

 specimen obtained, are only preserved towards the margins. The lines are 

 separated by rounded depressions of equal or slightly greater width. Length 

 of specimen 9 m, m., width on the hinge line 13 m. m. 



This specimen of Chonetes is undonbtedly different from any of 

 the three species of Chonetes, described in the preceding pages; but 

 since only a single specimen has been found, and that is not a very 

 perfect one, I have thought it best not to give it a name until better 

 materiiil has been obtained for illustrating the species. 



Tropidolcptus cairinatiis, Con. {S}).) Plate IX, figs. 1 and 9, and Plate X, 

 fig. 26. 



Strophomena carinata, Con. Ann. G. R. of N. Y., 1839, p. 64. 



Leptnena laticosta, Hall, 1843. 



Leptaena laticosta, of Owen and others. 



Tropidoleptus carinatus, Hall, lOtli Rep. St. Cab. N.Y., 1857, p. 151. 



Genus Tropidoleptus, Hall, 12th Rep. St. Cab. N. Y., 1859, p. 31. 



Leptaena laticosta, of several European geologists. 



Description of Erere forms : 



Test of medium size, plano-convex, transverse, semi-elliptical in outline, 

 sometimes slightly straightened in front, with the breadth about one and a 

 third to one and a half times the length, and greatest along the hinge line 

 which is straight. Cardinal angles slightly acute. Surface plicate. 



Ventral valve moderately convex, most elevated midway between the beak 

 and the center, whence it curves rapidly backwards, sloping to the anterior 

 margin along the middle with a very gentle curvature. From the flattened car- 

 dinal angles, the surface rises gradually on each side, being slightly concave for 

 a varying distance (one-half the width of the side or less), when it curves regu- 

 larly across the median line, very strongly on the posterior half, but less and 

 less so anteriorly. There is thus formed behind the center a sort of undefined 

 prominence, that broadens rapidly towards the front, gradually flattening out 

 and blending with the general curvature of the valve. Beak small, quite 

 strongly arcuate, and slightly extended beyond the hinge line in the interior 



