FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 131 



beaks of both valves almost touching each other. The length of the hinge line 

 is so variable that in some specimens it almost equals the greatest width of 

 the shell, while in others it is so much reduced that it, in connection with the 

 rounded extremities, and beaks touching each other, leaves its generic position 

 doubtful. Such specimens show nearer relation to Atrypa than to Spirifera. 

 The triangular lissure is also variable ; sometimes it is broad in the base, some- 

 times long and narrow ; in some specimens it is open, in others partially or 

 entirely closed by a pseudo-deltidium. 



Dorsal valve is almost as gibbous as the other valve ; its greatest convexity 

 is below the umbo, from where it regularly but gently curves to the anterior 

 and lateral margins. The mesial fold is arcuate from beak to front, pretty 

 well defined in its upper portion, but very slightly or not at all towards the 

 front ; it is rounded above the middle of the valve, but flattened and sometimes 

 even a little depressed in the basal half ; beak incurved over the linear area. 



Surface marked by regular fine radiating striae, which cover also the mesial 

 fold and sinus, some of which bifurcate or dichotomize towards the front. 

 Prof. Hall, in his description of this species in the Indiana Report, states, that 

 of these striae, eight or more occupy the space of a line, that the radii are 

 flattened, and that the interspaces are only about half as wide as the striae. 

 The specimen before me, the same figured on plate 29, which I found in the 

 quarry near the new water- works, east of Louisville, and which shows the 

 surface-markings as perfect as possible, agrees as to the number of striae in 

 the space of a line, but it differs as to the balance of Prof. Hall's observation. 

 In my specimen the striae are not flattened, but plainly rounded, and the 

 interspaces are certainly as wide, if not wider, than the radii. It appears to 

 me that the Waldron specimens differ in some respects slightly from those of 

 our strata. I have a good many Waldron shells before me ; they show finer 

 and closer set striae than the Kentucky specimens. 



In size this species is as variable as any other Spirifera, or even more so. 



In the quarries near our city we generally find the Spir. radiata of small 

 dimensions, some measuring only three -eighths of an inch in length and one- 

 half of an inch in width, while the specimen figured is about one of the 

 largest found near Louisville, and specimens of this size are extremely rare. 

 In the Niagara limestone near Waldron, Ind., we find specimens of an inch 

 and a quarter in length by a width of an inch and a half. 



Formation and Locality. Found in the Niagara limestone east of the city of Louisville, and on 

 the opposite bank of the Ohio river in Indiana. But the finest and most perfect specimens of this species 

 are found, even abundantly, near "Waldron, Indiana. 



