SPIRIFER^ OF THE UPPER HELDERBERG GROUP. 197 



Spii'ifera oweni. 



PLATE XXIX. 



Spirifer owcni : Hall, in Tenth Report on the State Cabinet, p. 129. 1859 

 " " Description of Now Pal. Fossils, p. 89. 1857. 



Shell more or less ventricose in its diflfereut stages of growth, somewhat 

 transversely oval, semielliptical or subquadrate : hinge-line about 

 equal to the width of the shell ; cardinal extremities rounded or sub- 

 angulated. Surface plicated. 

 Ventral valve scarcely so gibbous as the dorsal valve, its greatest con- 

 vexity above the middle of its length, and curving regularly to the 

 margins : beak much elevated above that of the opposite valve, and 

 arching over the fissure, but scarcely incurved ; mesial sinus shallow, 

 concave, usually well defined and reaching distinctly to the apex. Area 

 high, concave, elevated and continuing to the hinge-extremities ; fora- 

 men large, reaching to the apex, and sometimes partially filled by the 

 thickening of the dental plates. 

 Dorsal valve the more gibbous, the greatest convexity in the middle, 

 and 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. Are* narrow, vertical or in the plane of the longi- 

 tudinal axis. 



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

 gular 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 margin. 

 In well preserved specimens, there are distinct radiating striae. In many 

 of the silicified specimens, 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 rostral portion quite solid. The dental plates, reaching 

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



