SPIRIFER.^ OF THE UPPER HELDERBERG GROUP. 199 



Shell large, ventricose, transverse, with the hinge-line usually less than 

 the width of the shell ; cardinal extremities rounded or truncate, hav- 

 ing a subelliptical or subquadrate outline ; mesial fold and sinus ex- 

 treme. Surface plicated. 



Ventral valve variably convex on the two sides, with a wide mesial 

 sinus, which is well defined in the upper part, becomes wider and 

 deeper and less distinctly defined in the middle of the shell, and is 

 produced in front into a long triangular extension ; gently or more 

 abruptly curving from the greatest convexity to the sides and cardinal 

 angles : umbo prominent, with the apex incurved over the wide tri- 

 angular fissure ; area extending to the cardinal angles, with the mar- 

 gin rounded except towards the extremities. 



Dorsal valve gibbous, highly elevated in the middle into a strong angu- 

 lar mesial fold, and curving from the sides of the fold to the margins 

 of the shell, except at the cardinal angles, where it is a little flattened 

 and projecting, so as to give a minute auriculate appearance; summit 

 of the mesial fold regularly arcuate from beak to base ; apex slightly 

 incurved over the narrow nearly vertical area. 



Surface, on either side of the mesial fold and sinus, marked by from 

 sixteen to twenty plications, about four or five of which nearest the 

 centre are dichotomous from below the middle of their length ; ribs 

 low, and rounded above, flattened below the middle, those towards the 

 margin very slender : the first ten or twelve ribs on each side occupy 

 the greater part of the valve. The entire surface is marked by delicate 

 concentric striae, which are often crowded into imbricating lamellose 

 lines towards the front of the shell. In very perfect specimens, these 

 concentric striae are papillose or fimbriated by fine radiating striae. 

 These fine surface markings, however, are usually nearly or quite 

 obliterated. 

 The interior of a ventral valve shows but a partial thickening of the 



shell in the rostral cavity : the teeth are strong and short; the dental 



plates spreading and margining the upper part of the ovate muscular 



