460 WISCONSIN NATURAL HISTORY SURVEY 
specimens is this thickening indicated, but it has been shown by Dr. 
R. Ruedemann’ that in certain Ordovician cephalopods “the slight 
constriction of the living chamber [shown in the cast of the interior] 
is largely due to a thickening of the shell in apertural direction, 
evidently a gerontic feature.’”’ It is possible that the exteriors of the 
Fic. 1.—Lateral view of an exceptionally well-preserved specimen of Gomphoceras 
wisconsinense N.S. The measurements from which the model (Fig. 2 was constructed 
were largely from this specimen. The original is from Milwaukee and is now in the 
Public Museum, Milwaukee. 
upper portion of the shells of G. wisconsinense and G. calvini were 
much less concave than the interior, or were even convex. 
A comparison of these restorations with the fossils shows that a 
“number of the specimens which appeared at first to be distinct species 
' Cephalopods of the Champlain Basin (1906), p. 503, Fig. 57. 
