512 Contributions to Invertebrate Palaeontology. 



the muscular imprint of the ventral valve, and the more strongly- 

 incurved beaks, are the only features that can be relied upon. 



Formation and Locality. — In the hydraulic limestone of the 

 Lower Helderberg group, at Greenfield, Ohio. 



Genus RETZI4 King. 



Retzia formosa. 



Plate V, figs. 15 and 16. 



Wuldhemiu formosa Hall, 10th Rep. State Cab., 1857, p. 8S. 

 Trematospira formosa Hall, Pal. N. Y., vol. 3, p. 215, pi. 36, fig. 2. 

 Rhjnchospira formosa Hall, Pal. N. Y., vol. 3, p. 485, pi. 95 A, figs. 7-11. 



Shell small, the specimens observed not exceeding five-sixteenths of an inch 

 in length, by about one-fourth of an inch or less in width ; elongate-ovate in 

 form, widest below the middle and narrowing at the beak on the ventral side, 

 the apex being slightly incurved. Valves highly convex, with a slight depres- 

 sion along the middle. Surface of the shell marked by about twenty-two 

 simple, round, radiating plications, two of which in the middle of the valves 

 are more slender than the others and depressed below their level, forming a 

 slight mesial sinus on each valve. 



The shell, or rather the impression of the shell of this species as 

 left in the rock, ai)pears to represent an adult specimen, but is very 

 much smaller than those of the Lower Helderberg group of New 

 York, or those of i?. evax in the Niagara group at Waldron, 

 Indiana, but possesses all the essential specific characters of the 

 species except in this one particular. The species as recognized in 

 the Silurian rocks of Perry County, Tenn., resembles exactly this 

 from Ohio, both in size and general characters. It has proven 

 hitherto quite rare, but might possibly be found in greater abun- 

 dance were it sought for ; the specimens noticed occurring on blocks 

 of stone selected for other fossils. 



Formation and Locality. — Lower Helderberg group (Waterlime 

 beds), at Greenfield, Ohio. 



Genus RHY]VCIIO:\^ELI.A Fischer. 



Rliyiiclioiiella liydraulica. 



Plate V, fig. 17. 



Rhjnchonella liydraulica Whitf., Ann. N. Y. Acad. Sci., March, 1882, p. 194. 



Shell rather smaller than medium size, transversely oval in outline and 

 ventricose in jirofile ; the dorsal valve being highly convex, and the ventral 

 somewhat depressed convex. Beaks small, not prominent or conspicuous ; 



