FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 139 



simple, rounded or sub-angular, radiating plications, all of which, with the 

 exception of a few near the lateral margins, extend back to the beak; they 

 increase gradually in strength and in the size of their interspaces towards the 

 front. The size and number of these plications is variable in different speci- 

 mens. 



The two shells illustrated on plate 34, of which one is shown in figures 21, 

 22 and 23, and the other in figures 24 and 25, are so different in form and in 

 the size and number of the ribs, that I would be justified to put them in dis- 

 tinct species ; but there are so many intermediate forms connecting these two 

 extremes, which compel me to place them both in the above named species. 

 These shells have a somewhat close resemblance to Zygospira modesta, and 

 are considered by some geologists to be merely a larger and robust form of 

 that species ; bat it differs from that little shell not only by its size, which is 

 often more than twice that of Z. modesta, but also by its convexity, which 

 often increases to gibbosity, and also by its general aspect. I have collected 

 several hundreds of this shell ; I found them of all sizes ; some very large, as 

 shown in the illustrations, and again others very small, not exceeding the Z. 

 modesta in size ; but even these small or young shells differed so much in their 

 whole appearance from Z. modesta, that I think it necessary to separate our 

 shell from Z. modesta and place it in a new species to be named Z. kentuck- 

 iensis. 



Formation and Locality. I collected this fine shell in different places in Oldham county, Ky., but 

 I found it in great abundance at Taylor's Station, in said county, on the Shelbyville railroad, in the shales 

 of the Hudson Kiver or Cincinnati group. 



Genus Streptorhynchus. King. 



Streptorhynchus, King. Monogr. of Perm. Fossils 1850. 

 Etymology: strepto, I bend or twist; rhynchus, a beak. 

 Copied from Hall's Pal. N. Y., Vol. 4, page 641867. 



The shells of this genus are semi-circular or semi-elliptical, concavo-convex 

 or plano-convex, and sometimes bi-convex. They are externally striated, with 

 rounded bifurcating striae, which are crossed by fine concentric lines ; and in 

 some forms the stronger striae are distant, with finer radiating and concentric 

 striae cancellating the intermediate spaces. The ventral beak is sometimes 

 produced and bent or twisted, and the fissure beneath the beak is closed or 

 partly closed by a solid deltidium, while the area is subject to great variations. 

 A narrow area often exists on the dorsal valve, but this is not a constant char- 

 acter. This genus is very closely allied to Strophomena, the most obvious 

 external character by which the former is distinguished being the irregular 

 twisting of the beak of ventral valve. 



