NATURAL SCIENCES OP PHILADELPHIA. 55 



cene, while Trigonoarca disappeared at the close of the Chalk 

 epoch. It combines the characters of Axin^ea and Idonearca 

 and connects these with CucuLLiEA. 



The American species have a very short posterior hinge area, 

 which is rather long anteriorly. 



Subgenus Breviarca. 



Shell short ; hinge area minutely striated across ; hinge line de- 

 scending at the ends ; cardinal plates minute, crowded. 

 T. perovalis, Plate II., fig. 4. 

 T. Saffordii, Gabb, Plate II., fig. 3. 



18Y2.] 



