SHUMARDITES. 137 



At diaineter of K! luiii. (one-half revolution more tli;ui nt 12 nun.) the 

 three secondary lobes have ceased to show their ori<>-in, and are ari-anged 

 in a gentle curve across the sides, no longer corresponding to the shape of 

 the original lateral. The whorls begin to be more highly arched, and the 

 stage corresponds to Sddstoccras. Tne septa at this stage are shown on 

 PI. Ill, fig. 9. 



At diameter of 22 mm. (one-half i-evolution more tlian at 16 mm.) 

 the ventral lobe begins to be divided unsymmetrically l)y a secondary 

 saddle or notch; the first lateral is aftected in the same way; the second 

 lateral becomes strongly mucronate; the third lateral tends to become 

 notched, like the ventral and first lateral. The saddles are all still rounded 

 at this stage; the form of the whorl is as before, and the shell is making 

 the transition from goniatite to ammonite, from the Grlyjihioceratidfe to 

 the Arcestidre; or from a distinctly Paleozoic type to an harbinger of the 

 Mesozoic era. The septa at this stage are shown on PI. Ill, fig. 10, and the 

 form of the whorl is shown on PI. Ill, figs. 5 and 6, which niay be con- 

 sidered as the end of the adolescent, or paraneanic, period. 



At diameter of 38 mm. (one-half revolution more than at 22 mm.) the 

 ventral lobe is deeply divided on the side toward the umbilicus; the first 

 lateral similarly divided, but revei'sed; the second lateral slightly tripartite; 

 the third lateral like the first, but reversed. The saddles are deeply con- 

 stricted, and the general appearance of the lobes is phylloid, and suggestive 

 of Waagenoceras and Hyattoccras, of which genera Shiunardites is proliably 

 the ancestor. The septa of this, the adult, stage are shown on PI. Ill, fig. 

 11, and the form on PI. Ill, figs. 12 and 13. 



The ancestry of Shumardites is clearly seen in its successive stages through 

 Gastrioceras, Paralegoceras, Schistoceras, and its transition at maturity into 

 the primitive Arcestes type. That it is primitive is shown b}" the fact that 

 it makes this transition at such a large size (20 mm. in diameter), while all 

 the Permian and Triassic forms make this transition shortly after their 

 larval period, and at a very small size, thus illustrating the law of accelera- 

 tion of development. 



No other species is known with which Shumardites simondsi may be 

 compared, for this type has not been found in the Paleozoic deposits of 

 other regions. 



Occurrence. — In the Upj^er Coal Measures, Cisco formation, Missoui'ian 



