LAMELLIBRANCHIATA. 441 



The suggestion of the variety ocddentalis came from specimens occurring in 

 western localities, preserving the shell with all its exterior markings, in which 

 condition it has not been found in the Corniferous limestones of New York nor 

 in Ohio, so far as observed. A comparison of a large number of specimens, in 

 all conditions of preservation, indicates no differences of character which are of 

 varietal importance. 



Formations and localities. In the Corniferous limestone of Western New York, 

 Canada West, Michigan and Ohio ; also in the limestone of the age of the 

 Hamilton group at the falls of the Ohio, and Clarke county, Ind. 



Paeacyclas lirata. 



PLATE LXXn, FIGS. 2-19 ; AND PLATE XCV, FIG. 19. 



Posidonia lirata, Conkad. Geol. Surv. N. Y., Ann. Rep., p. 116, pi. [no numbei'], fig'. 12. 1838. 



Luciva {Paracyclas) lirata (Conbad), Hall and Whitfield. Twenty-fourth Ann. Rep. N. Y. State Mus. 



Nat. Hist., p. 200. 1872. 

 In part Pararyclas lirata (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 72, 



figs. 1-19. 1883. 



Shell of medium size, sub-circular or broadly elliptical ; length a little greater 

 than the height ; margins regularly rounded. Cardinal line short, less than 

 half the length of the shell. 



Valves moderately convex below, becoming gibbous on the middle and 

 above. 



Beaks anterior to the centre, small, appressed, rising but little above the 

 hinge-line. Post-cardinal slope not defined. 



Surface marked by fine concentric striae, and by strong sub-angular con- 

 centric ridges, which are more or less sharply defined, depending upon the 

 condition of the specimen and the nature of the matrix in which the fossil is 

 imbedded. 



Ligamental grooves distinctly marked and only moderately divergent from 

 the cardinal margin. 



Four specimens measure respectively 15, 19, 25 and 28 mm. in length, and 

 13, 18, 22 and 25.5 in height. 



