IIYPOTHYRIS. 347 



polished, yet sometimes a little glossy or even slightly 

 nacreous, and of a black or sooty hue ; the surface, except 

 at the extremities, is radiated with simple and rather 

 closely disposed striae (which become stronger as they 

 recede from the umboues), besides being marked with more 

 or less distinct and numerous concentric lines of increase. 

 The valves are unequal ; the upper or larger one, which 

 above is compressed at the sides, is likewise somewhat 

 flattened below, on account of an expanded but very 

 shallow mesial hollow, which excepting at its termination 

 is but little prominent; the lesser or lower one is the more 

 swollen, and is neither indented nor ridged. The acumin- 

 ated beak, which is perforated by a narrow triangular 

 orifice (which is not entire), leans very considerably over 

 the nestling and tumid umbo of the lesser valve. The 

 upper edges of the larger valve are much produced, and 

 decline so profoundly as to run at acute angles to each 

 other. The marginal outline (which is not crenated) is 

 peculiarly sinuous, the edge of the perforated valve indent- 

 ing that of the entire one on both sides near the umboues, 

 and likewise along the course of the mesial hollow. The 

 internal cardinal appendage consists of two recurved not 

 very long blades, that project, one on either side, from 

 beneath the umbones. 



The animal of this remarkable Brachiopod has been 

 examined by Professor Owen. He states that from the 

 two small processes, constituting the skeleton in this 

 species, and continued from the sides of the hinge of the 

 imperforate valve, " two spiral arms arise, fringed on 

 their outer margins, but quite free excepting at their 

 origins. When contracted they are disposed in six or 

 seven spiral gyrations, decreasing towards their ex- 

 tremities ; and, when completely unfolded, they extend 



