416 GEOLOGY OF OHIO. 



and straight, extending four-fifths of the length of the shell behind the beaks and 

 characterized by a narrow but distinct escutcheon. Anterior end short and full, 

 very obtusely pointed at the longest part, which is at about the middle of the 

 height, above which point there is a very distinct but narrow lunule extending to 

 the extremity of the hinge-line. Basal margin of the valve very broadly curved, 

 slightly emarginate just anterior to the middle and the whole subparallel to the 

 cardinal line. Posterior extremity sharply rounded below and the upper margin 

 very obliquely truncated ; body of the valve marked by a broad, distinct, mesial 

 sulcus extending from behind the beak to the broad sinus of the basal margin. The 

 umbonal ridge is rather sharply marked and angular in the upper portion, but be- 

 comes less distinctly marked posteriorly; postero-cardinal slope of moderate width, 

 very slightly concave in the 3'ounger stages of growth but less strongly marked as 

 the growth advances. Surface of the valves marked by strong, sublamellose, con- 

 centric lines of growth parallel to the outer margin of the valves. 



The shell undergoes considerable change in form and in the strength 

 of the surface characters between the younger and more advanced stages 

 of growth; the sharpness of the features being much reduced on the 

 older portions, by the rounding of the unbonal ridge and of the angular- 

 ity of both the anterior and posterior extremities of the shell. The shell 

 differs in several of its external features from the genus Modiolopsis, pos- 

 sessing a distinct lunule and escutcheon as well as the angular unbonal 

 ridge, in all of which features it corresponds with Goniophora. 



Formation and Locality. — In the hydraulic limestone of the Lower 

 Helderberg group at Peach Point, Put-in-Bay Island, Lake Erie, and at 

 Midcfretown, Marion Co., Ohio. 



ARTICULATA. 



CRUSTACEA. 

 MEROSTOMATA. 



Genus EURYPTERUS DeKay. 

 Eurypterus Eriensis. 

 Plate I, figs. 31, 32. 

 Eurypterus Eriensis, Whitf., Ann. N. Y.', Acad. Sci., March, 1S82, p. 196. 



Among the fossils from the hydraulic limestones of Peach Point, 

 Put-in-Bay Island, Lake Erie, there are several detached cephalic shields 

 and one body, of a species of Eurypterzis, which is so distinctly different 

 from any of those described, that it seems necessary to class it as a 

 separate species. The differences, so far as seen on the parts preserved, 

 consist in the form of the cephalic plate, in the size and position of .the 

 eye-tubercles, and in the proportions of the body as compared with the 

 known forms. There are undoubtedly other and more important differ- 

 ences in the appendages, but as these are not preserved on any of the 

 individuals examined, comparison is impossible. 



The cephalic shield is proportional \y broader than that of E. remipes 

 or E. laacs. ris, and is more regularly rounded or arched on the anterior 



