42 THE AMERICAN NATURALIST. [VoL. XXXV. 
cross-section. The surface is corrugated with wrinkles or 
curved ribs parallel with the striz of growth, forming a ven- 
tral, shovel-like extension of the aperture. The septa are com- 
plex, consisting of a divided ventral lobe, two pairs of laterals, 
and a short dorsal lobe. These resemble the septa of Lytoce- 
ras, but are simpler in digitation and number of lobes and 
saddles. Specimens of the mature shell are known nearly a 
foot in length, with scarcely any tapering of the form, so that 
the extreme size of maturity or old age must have been 
considerably greater than this. 
The phylembryonic or protoconch stage is very much like 
that of all the other angustisellate ammonites, except that the 
spheroid tends to become more angular, and the internal sep- 
tum begins to show traces of lobes and saddles. The siphonal 
caecum is unusually large, and was seen to be within that part 
of the protoconch cut off by the first septum. The limits of 
the embryonic body chamber were plainly seen on several 
specimens, marked by a constriction between the first and 
second septa, but not following the outline of either; the 
diameter at this stage was 0.53 mm. (Fig. 5). 
The next step in growth was the formation of the czecum, 
followed very soon by the development of the first septum ; this 
marks the beginning of the larval stage, as shown in Figs. 3. 
and 5. The body chamber of the first or ananepionic larval stage 
consisted of an entire revolution; thus the metamorphosis of 
the young animal must have been considerable. The surface 
of the shell in the phylembryonic and ananepionic stages was 
covered with pustules, giving a granulated appearance to it; 
but at the end of the first revolution these pustules ceased 
sharply at a constriction, and gave place to cross striz and 
ribs (Figs. 12 and 18). 
The second septum, which marks the beginning of the 
metanepionic stage, has a divided ventral lobe, and the full 
number of lobes and saddles that the animal possessed through- 
out life; the later changes consisted merely in the gradually 
increasing digitation of the septa, which, however, persisted in 
the goniatite stage not only throughout the entire coil, but also 
for two and a half millimeters of the straight shaft (Fig. 6). 
