FROM THE CRETACEOUS FORMATION OF NEBRASKA. 395 



auxiliary lobe. Superior lateral lobe extremely contracted in the middle by the 

 lateral branches of the saddle ; divided towards its extremity into three unequal 

 branches, the terminal one trifid at its extremity, the lateral ones scarcely digitate ; 

 two smaller lateral branches towards the base. Lateral saddle in form like the dorsal 

 saddle, with the ventral division larger and bipartite, corresponding to the dorsal di- 

 vision of the other. Inferior lateral lobe shorter than the superior ; contracted near the 

 middle, divided into three sub-equal branches, the lateral ones irregularly digitate, 

 and the terminal one trifid. Ventral saddle oblique, divided by the auxiliary lobe 

 into two branches, which are again bifurcate, with the extremities obtusely bifid. 

 Ventral lobe much smaller and shorter than the inferior lateral lobe, sub-equally tri- 

 partite, with the divisions sub-digitate. A small bilobed saddle on the ventral side of 

 the last lobe. 



The characters here given are derived from a small specimen, (PI. IV. fig. 1, «,) and 

 from the inner volutions of an older one, (fig. 1, h, c,) while in the outer volutions of 

 the same specimen the lobes and saddles become very much crowded together, and 

 exceedingly complicated in their structure ; the division in the dorsal lobe becomes 

 much deeper, all the divisions already noticed are more complex, the sinuosities ex- 

 tended in depth, the simple digitations become complex ramifications, Avith each 

 division again sinuous on the edges, illustrating in a remarkable manner the devel- 

 opment of this complicated structure as the animal increases in age and dimensions. 

 In the young specimen figured, the septa in the interior volutions present the sim- 

 plicity of those of Goniatites ; while the outer septa of the same specimen exhibit the 

 structure described and figured. 



In the older specimen, (PI. IV. fig. 1, b,) the outer septa furnish the extremely 

 complex structure given in figs. 1, e, and 1, /, Avhich include the dorsal lobe and a 

 part of the dorsal saddle, as well as the superior lateral lobe, disconnected from the 

 preceding parts ; this being as far as the imperfection of the specimen and the ex- 

 treme complication of the structure would allow one to follow its divisions ; while 

 the inner volutions present precisely the same structure as the outer volution of the 

 younger specimen. 



This species resembles in general form and proportions the A. Mantellii of Sowerby, 

 but the annulating ridges are less strong, and the nodes on the inner edge very dis- 

 tinct. There is a greater dificrence, however, between the two species in the form and 

 details of the lobes of the septa. 



Locality and Position. — Great Bend on the Missouri. Lower part of division No. 4 

 of Section. 



