58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Table 7. — Measurements of Atribonium pingue from the Four Mile Dam Forma- 

 tion near Four Mile Dam, Alpena County, Mich. 



Measurements, in millimeters 



Comparisons. — Atribonium pingue is characterized by its large size, 

 comparatively great convexity, normally rather gently curved anterior 

 geniculation, outwardly convex anterior surface, and its rounded 

 costae that begin far forward. Some specimens of this species attain 

 rather large size before beginning the right-angle growth at the anterior 

 that produces the great thickness and the wedge shape of most speci- 

 mens (longest known specimen maintaining juvenile form: 10 mm.). 

 Only two known species of Atribonium attain a maximum size as great 

 as that of A. pingue; these are A. simatum n. sp. and A. gregeri (Bran- 

 son) . The latter is very convex, normally globose, and has a low fold. 

 A. pingue differs from A. simatum in its fewer and weaker costae, 

 higher fold, greater thickness of adults, and proportionately somewhat 

 narrower outline. 



Fig. 10. — Scatter diagrams of dimensions of Atribonium pingue from Four 

 Mile Dam, Mich. 



