866 The American Naturalist. [October, 
complete fossil cast of a full grown Metatoceras cavatiformis 
Hyatt, and some of the lines or sutures made in the external 
surface of the cast by the intersections of the partitions or 
septa that cut up the coiled tube of the living shell into air 
chambers. Figs. 3-4 show a broken specimen of the same 
t 
Fic. 3. Fic. 4, 
species, but with the outer and older whorls in large part 
removed. The innermost septum near the center of the coil 
was built across the interior after the animal had constructed 
the hollow apex or point. It then moved along adding to the 
external wall of the tube, which has been destroyed and 
removed from this cast, and built the second septum, and so 
on until it reached the tenth septum. By some freak of fossil- 
ization a number of the septa beyond this have been 
destroyed, so that if we were to remove the fragment of the 
external whorl and take out the center which has just been 
described, this would have the exact aspect of a cast of a 
young shell with ten air chambers.’ The eleventh air space or 
chamber being open and without divisions would then appear 
to be the living chamber which the animal occupied when it 
built the tenth septum. Normally the shell really continued 
to progress from the tenth septum by additions to the outer 
* The shaded area in the center, shaped like a large inverted comma, was an 
me iei x Hw living me eS is pns desint pie icd z% ent matrix in 
as in ee als so 
figs. 4, 5, 6, on PI. XVIII, which show the comma Rein umbilical ceste or 
openings left at the center through the crytoceran form of the young. 
