PLATE VI. 



PAGE 



Figs. 1-2. Mytilarca percarinata Whitf 422 



1. Vertical view of a left valve, the extremity of the hinge is restored in outline 



2. View of the anterior side of the shell showing the depth of the shell. 



Figs. 3-4. Trematoceras Ohioense Whitf. 426 



3. Lateral view of the specimen used for description showing the line of nodes 



formed by the closing of the perforations, the upper one being in the outer 

 chamber was not entirely closed. ' 



4. Section of the lower »nd of the specimen showing the form of the siphon. 



Figs. 6-7. Gomphoceras Sciotense Whitf 428 



6 and 7. Fr nt and lateral views of the specimen showing the form and the septa. 

 For other figures see Plates VIII and IX. 



Fig. 8. Gyroceras Columbiense Whitf 430 



8. Lateral view of an imperfect internal cast of a specimen of about the usual size 

 showing indications of the arrangement of the septa. 



