PLATE XII. 



Pentamerus oblongus, Sowerby, var. subrectus, Hall. 



Page 368. 



Figs. 1, 2. Dorsal and ventral views of a normal mature individual; showing 



the characteristic outline, prominent umbo, trilobate exterior and linear 



median depression on both valves. 



Fig. 3. Dorsal view of a large individual, slightly imperfect at the anterior 

 margin. 



Fig. 4. A somewhat weathered specimen, in which the valves have been dis- 

 placed from their normal position, exposing the spondylium of the pedicle- 

 valve, and, by the removal of the rock, also showing part of the united 

 septal plates of the brachial valve . 



Figs . 5, 6 . Ventral and dorsal views of a small specimen of subquadrate out- 

 line, trilobed exterior, and showing the single median septum on each 

 valve . 



I Niagara beds. Jones county , Iowa . 



GyPIDULA ROMINGERIj Hall. 

 Page "369. 



Fig. 7. Cardinal view of a pedicle-valve; showing a well-defined cardinal 



area, the teeth and the form of the spondylium. 

 Fig. 8. Dorsal view of the umbonal region of the specimen represented in 



fig. 30; showing the sharply defined cardinal area, and the deltaria or 



remnants of the deltidi am . x 2. 



Fig. 9. Interior of a large brachial valve; showing the size and structure of 

 the spondylium . 



Fig. 10. A smaller specimen; showing similar features. 



Figs. 11, 12. Dorsal and profile views of a large specimen; showing the char- 

 acters of the exterior. The surface of the brachial valve is somewhat 

 exfoliated in the umbonal region, exposing the base of the spondylium 

 and the vascular markings . 



Fig. 13. An individual of average size with an unusually flat brachial valve 

 and coarse, duplicating plications. 

 Hamilton group . Near Alpena, Michigan. 



398 



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