FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 127 



curving regularly to the front and lateral margins, and usually a little flattened 

 or sometimes concave towards the cardinal extremities ; mesial fold prominent, 

 rounded, with a longitudinal depressed line along the middle. Area narrow, 

 vertical, or in plane of the longitudinal axis. 



The surface is marked by from fifteen to seventeen rounded or sub-angular 

 plications on each side of the mesial fold and sinus, and these are crossed by 

 distinct concentric striae, which become strongly imbricating, or are marked in 

 strong imbricating lines of growth towards the margins. In well preserved 

 specimens there are distinct radiating striae. In many of the silicified speci- 

 mens, however, both the radiating and concentric striae are partially or 

 entirely obliterated. The interior of the ventral valve shows two short and 

 rather strong teeth, with the rostal portion quite solid. The dental plates, 

 reaching to the bottom of the cavity of shell, curve slightly outwards and 

 partially inclose an oval muscular area, which in its upper part is divided by 

 a short medium crest. In some silicified specimens the conical spires are 

 partially and sometimes entirely preserved. The crura are widely separated 

 at their bases, and converging somewhat abruptly, curve into the dorsal valve, 

 making twelve or more turns, and producing a short strong spire. In well 

 preserved specimens the mesial fold and sinus are usually well defined, but in 

 some of the more gibbous forms the sinus is very broad, and one or two of the 

 plications on each side are involved in the sides of the depression, at the 

 same time the mesial fold is very prominent, rounded, and sloping almost 

 imperceptibly into the general contour of the convexity of valve. 



This species has been generally referred to Spir. laemcosta of Lamarck, from 

 which it differs in being less gibbous, having a greater number of plica- 

 tions, and having also a depressed line along the middle of the mesial fold of 

 the dorsal valve. It may be easily distinguished from Spir. euruteines, with 

 which it is associated, by its narrower and much more arcuate area and less 

 angular plications. Some specimens show remains or traces of a faintly im- 

 pressed line along the center of each plication. (Hall.) 



Formation and Locality. Found in great abundance and beauty in the Devonian limestone in 

 Kentucky and Indiana, around the Falls of the Ohio. Specimens showing the internal spiral coils well 

 preserved in silicified shells are not very rare. This species was named by Prof. Hal! in honor of D. D. 

 Owen, the former State Geologist of Kentuc' 



