256 SYNOPTIC COLLECTION. 



Finally, the straight form is very nearly attained in 

 Baculites (PI. 624, figs, i-io ; No. 625), but this straight 

 adult Ammonite has a tiny coiled shell when young. It 

 is interesting to note that the protoconch has become 

 modified, -so that its shape is suggestive of a spiral when 

 seen from the side (PI. 624, fig. i). The typical globu- 

 lar form has become broadly elliptical (fig. 2, front view) 

 with a projection extending forward (fig. i). 



The earliest stage of the larval nepionic shell is seen 

 in fig. 3. An older shell having four septa shows the 

 siphuncle near the center (fig. 4). The aperture at this 

 stage is broad, and the area of contact of the revolving 

 whorl upon the preceding whorl is also broad, as shown 

 by the dotted lines. The septa at first are simple. Fig. 

 5 represents a side view of the shell with six septa, and 

 fig. 6 the first six sutures. Three of these sutures are 

 simple, while the remaining three are similar to the 

 sutures of Goniatites. A front view of a stage with thir- 

 teen septa (fig. 7) shows the siphuncle near the margin, 

 the aperture and area of contact narrower so that the 

 growing whorl envelops less of the shell. In a still older 

 stage with seventeen septa (fig. 8) the siphuncle is close 

 to the edge, the aperture is almost circular, while the area 

 of contact is much narrower, as indicated by the dotted 

 lines. When the shell has the diameter of one millimeter 

 and consists of two or two and a half whorls with from 

 twenty to twenty-five septa it begins to grow out in a 

 straight line. Fig. 9 represents the adolescent (neanic) 

 shell at this stage with the lines of growth and the ros- 

 trum at the opening. 



The sutures, which are concealed in fig. 9, become 

 more complex, passing from the Goniatite condition to 

 the Ceratite (see lower suture in fig. 10) at about the 

 thirtieth septum or after the shell has become straight. 

 After this the lobes and saddles increase in number until 

 the extremely complex sutures of the adult Ammonite are 

 produced (upper sutures in fig. 10; see also No. 625). 



