ENGONOCERATID 2. 155 
7.5 mm., and opposite side same volution 46.5 mm., no shell present. The 
greatest transverse diameter of the volution is 30 mm., and of thesmaller 
part opposite is 22 mm. 
The shell is marked by bands of growth and fold-like obscure coste 
which appear in the gerontic stage. These terminate at the umbilical 
shoulders, and along the centran surface of the lateral aspect have very 
broad swellings and then subside into the flat general surface toward the 
periphery. The venter is slightly broader than in the ephebic stage above 
described and the volution stouter, owing to the development of lateral 
swellings and the slight decrease in the involution due to old age. The 
umbilical shoulders and the umbilical zones are abrupt instead of being 
rounded and sloping as in the adults. The living chamber is complete 
near the line of involution and is about one-half of a volution long 
internally and apparently about the same externally when restored. The 
concavity of the venter is maintained for one-half of the length of the 
living chamber. Beyond this it could not be followed, but there are some 
indications of the possible rounding of the venter in extreme age. 
The sutures are quite distinct from those of the ephebic stage in the 
specimen above described. There were eight saddles and seven lobes to the 
umbilical shoulders, remainder on the umbilical zones being concealed. 
The interesting fact, however, can be noted that in this gerontic stage new 
saddles and lobes were not added as the sides broadened. The increase of 
the sides was met by the broadening out of the saddles. The first laterals 
were very broad, the second and third had not changed much, but begin- 
ning with the fourth they became irregularly broader toward the umbilicus, 
and the seventh was 6 mm. in breadth, whereas the sixth lobe was only 
about 1 mm. long. The lobes remained about the same as in the adult 
stage. 
The form of the volutions, smooth concave venter, slightly costated 
sides, with large folds only in gerontic stage, and primitive sutures all show 
that this is a species like the similar stages in the development of tuberculated 
forms in Metengonoceras and also similar to the young of Placenticeras. It 
is, however, a deeply involute shell and is not therefore by any means the 
most primitive form of its own subseries. It indicates the existence of a 
distinct subseries having similar smooth concave venters and less involute 
or more discoidal shells, which in the gerontic stage become stouter with 
