224 AN INTRODUCTION TO THE STUDY OF FOSSILS 



marked by coarse concentric undulations. This genus is 

 especially characteristic of the Cretaceous. 



Fig. 93. — Inoceramus barabini Morton, from Montana. This pelecypod abounded 

 in the shallow waters of the large ocean which cut North America into an eastern 

 and a western continent during Cretaceous times. A, hinge view of entire shell. 

 The valves are separated posteriorly, and the beaks have been slightly displaced ; 

 the absence of articulating teeth renders such displacement, through the weight 

 of sediment, very easy. B, outer surface of left valve of same specimen. Natural 

 size of a small individual. 



1. Sketch {a) hinge view of combined valves ; {h) outer surface 

 of one valve. Label in sketches umbo, hinge line, growth 

 ridges, oldest portion of shell, youngest portion, right valve, 

 left valve. 



2. Distinguish between umbo and beak. 



3. What was the function of the row of pits along the hinge 

 line ? 



4. Account for the slipping of one valve past the other in 

 most specimens, so that the edges of the valves do not meet edge 

 to edge. This slipping seldom occurs in such forms as Unio and 

 Venus. Why ? 



5. How are pelecypod shells thickened? 



6. Is the shell external or internal ? 



Pteria. Ordovician to present. 



Shell oblique, inequilateral and inequivalved, with the left 

 valve more convex than the right. Hinge line long, bearing 

 one or two small cardinal teeth and a long lateral tooth. Pos- 

 terior ear wing-like, longer than the anterior. Sinus for the 

 passage of the byssus present under the right anterior ear. 



