FOSSILS OF THE CLINTON GROUP. 585 



in the outline of the shell. The beak is strongly developed. The shell 

 has the aspect of Meristella Pr his tana, or of some slightly more elongate 

 type, like Meristella intermedia, but it is covered with fine radiating striae 

 exteriorly, like those of Orthis elegantula in size. On closer examina- 

 tion it seemed possible to recognize the pitted structure of this section of 

 the genus Orthis, and so the specimens are probably an aberrant form of 

 Orthis elegantula, with at times several concentric lines of growth. In- 

 teriors have not been found, and these shells are mentioned only ior the 

 sake of completeness, and to indicate the difficulties at times attending 

 the identification of these Ohio shells, the hinge area being frequently 

 not shown, even on otherwise fairly preserved exteriors. 



If Orthis circulus, Hall, be indeed present in Ohio rocks, it can be 

 only represented by a few valves found at the very top of the Clinton at 

 Soldiers' Home, where it changes to the stratigraphical equivalent of the 

 Dayton limestone, the base of the Niagara, by a decrease in the amount 

 of lime, and the increase of the magnesium carbonate, the rock not being 

 here a true limestone. These valves resemble valves of Orthis hybrida, 

 but are larger, 19 mm. long, and the anterior half of the pedicle valve is 

 not so much flattened, remaining slightly convex in all directions. What 

 was regarded as the brachial valve was similar, with the posterior half of 

 the surface, especially near the beak, more convex than the rest of the 

 valve. It will be impossible to identify these forms more accurately 

 until better specimens are found, some showing the two valves in posi- 

 tion, and others showing the muscular scars and the hinge area. 



Triplecia Orto?ii, Meek. 



In the initial stages of growth this species has both valves nearly 

 equally convex. In specimens 10 mm. long the pedicle valve has be- 

 come more flattened than the brachial valve, the latter having become 

 strongly convex. This difference is increased by the appearance of a 

 broad shallow median sinus in the pedicle valve, and a less distinct me- 

 dian fold in the brachial valve. As growth increases the sinus becomes 

 deeper, broader and more distinct. In mature specimens the sinus forms 

 a rather triangular area, the general surface of which is more or less 

 equally concave posteriorly, but anteriorly the median region is often 

 quite flattish, the sides of the sinus rising more abruptly to the general 

 surface of the valves, giving thus more distinctness to the rounded 

 lateral boundaries of the sinus. This sinus is strongly convex antero- 

 posteriorly. Leaving however the sinus out of account, the pedicle 

 valve is seen to be much flatter, in mature shells, than the brachial valve. 

 In forms with a long hinge area this flattening is much more marked 

 than in those with a short hinge area. The median fold increases in 

 height and breadth with age. Owing to its broad lateral slopes anteriorly, 

 it is not so easy to appreciate the full degree of increase in width of the 

 fold as the increase in width of the median sinus. The median fold has 



