48 Trans. Acad. Sci. of St. Louis. 



hinge line is gently concave, anterior to the beaks the 

 shell is imperfect and the characters of the hinge can not 

 be made out. Anterior margin regularly rounded from 

 the extremity of the hinge line to the basal margin, basal 

 margin convex anteriorly but becoming nearly straight 

 near the posterior extremity of the shell. A sharp narrow 

 ridge extends from the beaks to near the postero-basal 

 angle of the shell, its most prominent part descending at 

 an angle of thirty-eight degrees with the hinge line. 

 Near the beaks the ridge is almost straight and very low, 

 but it increases in height as it passes downward and back- 

 ward. For the lower two-thirds of its length it is high 

 sharp, and convex upward. Anterior to this ridge is a 

 narrow sulcus which begins about three millimeters from 

 the beaks and increases in depth and width as it passes 

 downward. Posterior to the ridge is a broad sulcus 

 slightly arched over by the ridge. On the anterior two- 

 thirds of the shell the surface is marked with fine concen- 

 tric lines. At the anterior edge of the sulcus in front of 

 the ridge these lines meet coarser lines of the sulcus at 

 an angle of about eighty degrees. Posterior to the ridge 

 short, ill-defined lines descend into a sulcus where they 

 meet at a sharp angle lines passing downward and back- 

 ward from the hinge line. 



This species differs from Tedinopliorus divaricatus, 

 the nearest allied species, in being proportionally much 

 broader, in the sulcus anterior to the ridge being nar- 

 rower and starting from a point posterior to the beaks, 

 in the ridge being sharper, concave upward, arched over 

 the posterior sulcus, and forming a greater angle with 

 the hinge line. 



This genus has usually been classed with the pelyco- 

 pods, but the characters of the hinge are very different 

 from those of any known pelycopod and it should proba- 

 bly be classed with the bivalve crustaceans. 



Species based on a single well preserved specimen. 

 Number 11551 of the invertebrate paleontological col- 

 lection of Walker Museum. 



