FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 41 



valve moderately convex, most elevated at between one-fourth and one-third 

 the length from beak, thence descending with a flat or gently convex slope to 

 the lateral margins, with a somewhat concave one to the front, and also to 

 hinge-line and cardinal angles. The concavity towards the front is not found 

 in all specimens ; some shells have basal portion either flat or slightly convex. 

 The ventral umbo is small and neatly defined ; beak small, pointed, and some- 

 what incurved, but scarcely overhanging edge of area ; area triangular and 

 somewhat larger than the dorsal one. Foramen not observed. Surface covered 

 with small sub-angular radiating ridges or striae, of nearly uniform size, from 

 eight to ten in the width of three lines, increasing by bifurcation, strongly 

 curved outwards on lateral parts of shell ; the interspaces sub- angular and 

 equal in size with the striae. 



"In very perfect specimens very fine concentric, sub-lamellar striae are visible, 

 seven or eight to one line. In certain conditions of preservation, also, the 

 radiating striae are seen to be sub-tubular, and exhibit numerous small oval 

 or circular openings on their edges, each about the eighth or tenth of a line in 

 width, and from one-fourth to two- thirds of a line distant from each other." 

 (Billings.) The largest specimen which I have seen of this species is the one 

 illustrated on plate 16, figures 23 and 24, though, according to Mr. Billings' 

 statement, this species attains in some specimens a considerably larger size. 



This species is allied to Orthis vanuxemi, but is more coarsely striated. 



Formation and Locality. Found in the rotton hornstone in the upper strata of the Devonian 

 formation, in Jefferson county, Kentucky, and in Clark county, Indiana. It is rarely found in specimens 

 of the size as shown in the illustration, while smaller shells, like those figured plate 17, figures 33, 34 and 

 35, are not so rare; with these latter there is, however, the trouble to distinguish them from smaller 

 specimens of vanuxemi. 



Orthis linneyi. w. SP. 



Plate XXXIV., figures 7 to 13. 



Shell of medium size; sub-circular or sub-elliptical; width exceeding 

 length; hinge-line short, measuring little more than one-third the width of 

 the shell ; cardinal extremities broadly rounded ; lateral margins regularly 

 curved ; basal margin broadly curved, showing in the dorsal view a slight 

 emargination, and in ventral view a small basal extension in the central part 

 of the front. (Figure 9, on plate XXXIV., shows a basal extension, where it 

 should show a small sinus or inflection.) In regard to depth, it varies from 

 moderately convex to gibbous. 



Ventral valve is mostly less convex than the opposite one ; its greatest con- 

 vexity is at the umbo, from where it slopes in a more or less curved line to 

 baso-lateral margins, and more abruptly to the cardinal borders ; central part 

 of valve is depressed, forming a mesial sinus, which extends from beak to 



GEOL. SUR. 6 



