136 CARBONIFEROUS AMMONOIDS OF AMERICA. 



deeply embracino^, covering two-thirds of the inner vohxtion, and indented 

 to one-half of its height by the inner whorl. Abdomen broadly rounded 

 in a curve that extends almost unchanged to the angular and steep umbilical 

 shoulders. Umbilicus about one-third of the total diameter, deep, and 

 exposing the umbilical shoulders of the inuei' whorls. 



Surface smooth, so far as could be observed on the cast, except that a 

 few constrictions are visible, more common on the inner whoi'ls. 



Septa of the lanceolate type, saddles rounded and constricted, lobes 

 also constricted, and pointed. Ventral lobe divided by a notched siphonal 

 saddle not quite so broad as the two divisions of the lobe. The ventral 

 lobe is unsymmetrically divided by a deep cleft or adventitious saddle; 

 the first lateral lobe is smaller than the ventral and is divided in the same 

 unsymmetric way; the second lateral lobe is slightly larger than the first, 

 and is mucronate in shape, showing a tendency to become tripartite; the 

 third lateral lobe is similar to the first, except that it is the reverse of it. 

 A fifth lobe, or auxiliary, sharply pointed and rather long, stands on the 

 umbilical shoulder. 



The internal septa consist of a tripartite antisiphonal lobe, a bifid first 

 lateral, and a simple second lateral, with a small auxiliary on the umbilical 

 border. 



The siphonal saddle is nearly as bi'oad as the ventral lobes, the first 

 lateral saddle is still broader, while the second and third laterals are nar- 

 row; the fourth lateral, just above the umbilical shoulder, is broad and 

 short. 



The lobes are not arranged in a straight line across the sides, but in a 

 backward-pointing curve, which would suggest that the lateral lobes are 

 secondary, and have developed out of a single primary lobe; this is borne 

 out by the ontogeny of the species. 



Ontogeny. — At the diameter of 7 nnn. the whorl is depressed, trapezoidal, 

 and scai'cely arched. The cast is marked by frequent deej) and cur^'ed 

 constrictions, and the septa, as shown on PL III, fig. 7, are gastrioceran, 

 although the lateral lobe is beginning to become tripartite. This is the 

 gastrioceran stage. 



• At diameter of 12 mm. the ventral lobes have liecome much longer, 

 and the lateral is divided into three nearly equal secondary lobes; at this 

 stage the whorl is more highly arched, and the stage is transitional from 

 Paralegoceras to Schistoceras. The septa of this are shown on PI. Ill, fig. 8. 



