590 GEOLOGY OF OHIO. 



median ridge, traverse the anterior part of this posterior impression, be- 

 ing most distinct near its anterior border and disappearing rapidly poste- 

 riorly. In one specimen as many as ten of these low ridges form a lunate 

 arc occupying the anterior part of this impression. The general form of 

 the posterior adductor scars is obliquely oval. The anterior adductor im- 

 pressions are well defined only along their posterior and inner margins 

 as described above. In one specimen the edges of the median ridge 

 separating them seem fairly parallel almost to the anterior end of the 

 anterior adductor impressions, but in one case this grooved median ridge 

 begins to separate about on line with the middle of these impressions, 

 the divisions curving strongly outwards laterally and becoming faint so 

 rapidly that the continuation of this ridge into a slight border, caused by 

 the thickening of the shell which bounds the sides of the anterior impres- 

 sions and connects with the lateral ridges defining their posterior margin, 

 was perhaps rather suspected in one specimen than definitely deter- 

 mined. No markings seem to characterize the area of these anterior 

 adductor impressions. The impressions are therefore distinctly bounded 

 only posteriorly and along their inner margin; the} T are obliquely oval in 

 form. 



This species is found at the Soldiers' Home, Fauver, Huffman, Cen- 

 terville quarries in Ohio, in the upper shaly courses, rarely in the lime 

 stones. 



Mcristella umbonata, Billings. 



(Plate 25, Figs 2, a, b.) 



The absence of interiors of this and other meristelloid shells in Ohio 

 makes it not only impossible to determine accurately their generic re- 

 lations, but casts considerable doubt upon their specific identification, no 

 matter how carefully made, by means of their exterior characters. 

 Nothing has been added to our knowledge of Ohio forms of the species 

 identified as Meristella umbonata, Billings. After a careful examination 

 of New York Clinton and Niagara forms of this and related genera, the 

 Canadian species, above mentioned, still seemed to be the most nearly- 

 related form. The even curvature of the lateral outline, the total 

 absence of antero-lateral angulation, the very moderate curvature of the 

 line of junction of the two valves anteriorly, and the vfery even though 

 elongate convexity of the shell, seemed to be the characteristic features. 

 The surface of the shell is marked by fine concentric striae of growth. 

 The exfoliated pedicle valve of a small specimen left a matrix which 

 indicated an elongate deep delthyrium slightly constricted anteriorly at 

 its base, then widening out anteriorly into the muscular area, which 

 shows six to eight moderately radiating striae, and a shallow median 

 groove. This form is found at Soldiers' Home, Ohio. Single valves are 

 found at Huffman's Quarry and at the Soldiers' Home in Ohio, which 



