PALEONTOLOGY. • 415 



valves has been destitute of plications, but is usually marked in the larger indi- 

 viduals by several strong varices of growth near the front margin, which give to 

 the shell a prematurely old appearance for so small a species ; the individuals seldom 

 exceed five-eighths of an inch in length on the ventral side. 



•This species is unlike any known form of a similar size, in the shal- 

 lowness of the valves, in the erect character of the ventral beak, and in 

 the deeply divided feature of the cast of this valve. The dorsal valve is 

 much less marked, and is often destitute of any distinguishing feature. 



Formation and Locality. — In the hydraulic limestone of the L,ower 

 Helderberg group, in Adams county, Ohio, occurring in numbers densely 

 packed together, but having the shelly substance entirely removed. 



MOLXUSCA 



LAMELLIBRANCHIATA. 



Genus PTERINEA Goldfuss. 



Pterinea aviculoidea. 



Plate I, fig. 23. 

 Megambonia aviculoidea Hall, Pal. N. Y., vol. 3, p. 274, pi. 49, figs. 7 and 8. 

 Pterinea aviculoidea (Hall) Whitf., Geol. Rept. Wis., vol. iv, p. 322, pi. 25, figs. 6-7. 



Shell of proportionally small size, obliquely rhomboidal in outline, with a 

 moderately long, straight hinge-line, but little shorter than the length of the body 

 of the shell below. Left valve depressed convex, with a small, slightly incurved 

 beak, scarcely extending above the cardinal line. Anterior end "fhort, and the an- 

 terior projection scarcely defined ; posterior wing concave and the posterior margin 

 nearly at right angles to the hinge-line for a short distance below, then gently curv- 

 ing backward to the rounded postero-basal extremity; basal line rounded and on 

 the anterior side of the umbonal ridge curving rapidly upward to the anterior ex- 

 tremity. Body of the valve convex and oblique to the hinge, the umbonal ridge 

 broadly rounded. Surface of the shell marked only by lines of growth some of 

 which are stronger and form slight varices. 



The'species is poorly represented in individuals, but the specimens 

 seen are so precisely like those of the Tentaculite limestones of New 

 York as to be not readily mistaken. 



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

 Helderberg group on Put-in-Bay Island, I^ake Erie. 



Genus GONIOPHORA Phillips. 



Goniophora dubia. 



Plate L figs. 24-26. 

 Modiolopsis? dubius Hall, Pal. N..Y., vol. 3, p. 264, pi. 49, fig. 2. 



Shell. small, transversely elongate, nearly twice and a half as long as high. 

 Valves ventricose, most highly convex on the anterior half, becoming more de- 

 pressed toward the posterior ; beaks small, very slightly incurved but not elevated 

 above the cardinal border and rather inconspicuous, situated about half or rather 

 less than half the height of the shell from the anterior extremity, proportionally 

 more distant on the larger specimens than on those of small size. Hinge-line long 



