PTTEBECK BEDS OF THE VALE OE WARD OUR. 



247 



anterior side rounded and somewhat steep; the base (inferior or 

 ventral border) flattened ; the postero-dorsal (nmbonal or superior) 

 border concave and much elongated. 



Tig. 1. — Tritjonia densinoda, Ether. 



Umhones pointed, small, but prominent or elevated and slightly 

 recurved, antero-mesial, or placed within the anterior third of the 

 valves. 



Area moderately wide, flattened and filled with reticulated costellae, 

 which are minutely and delicately nodulated or tuberculated ; it is 

 bounded by two carinae, the outer densely or closely nodulated, in- 

 creasing in width from the umbo to near the extremity of the pos- 

 teal side, then merging into the system of tuberculated transverse 

 costse ; or the extreme posteal border possesses many irregular lines 

 of growth due to the elongation of the tubercles. The inner 

 carina, or that bordering the escutcheon, consists of a series of minute 

 tubercles which are immediately surmounted by the tranverse wavy 

 ridges of the escutcheon. The median sulcus, or mesial furrow, 

 is well defined. The escutcheon is remarkably large, having trans- 

 verse wavy rugse resembling those occurring in the INeocomian Qua- 

 dratse. The shell (valve) possesses regularly and concentrically 

 arranged tuberculated or knotted rows of costae ; about 40 range 

 from the umbo to the posteal attenuated border. Those tubercles 

 upon the posterior third of the valve, below the area become elongated 

 and less regularly arranged than those of the umbonal region, where 

 they are closely concentrically arranged. 



Affinities and Differences. — In outward ornamentation T. densinoda 



s2 



