INVERTEBRATE PALEONTOLOGY. 415 



3. discoscaphites, Meek. 



a. Shell with general outline subcircular, or slightly oval, and 

 generally much compressed ; inner volutions forming a large part of 

 the entire bulk, and so deeply embracing as to leave only a small 

 umbilicus ; body-portion so very short, as scarcely to become free at 

 the aperture, flattened on the periphery ; surface ornamented with 

 costse, and provided on each side with from about four to nine rows 

 of tubercles, the outer of which are largest and arranged along each 

 margin of the periphery. — (Scapkites Conradi, Morton.) 



b. Shell differing from the last chiefly in having the volutions so 

 narrow and little embracing as to leave a large shallow umbilicus, and 

 the body-volution deviating very little from the regular curve of the 

 others. — {Ammonites Cheyenncnsis, Owen.) 



The most marked distinction between the genus Scapkites and the 

 genus Ammonites, as the latter is understood in its original comprehensive 

 signification, consists in the deflected and more or less extended and recurved 

 body-portion of the former. In Scapkites, however, there is not near so great 

 a diversity in the mode of division of the septa into lobes and sinuses, as in 

 the unrestricted old genus Ammonites. Still, there seems to be no means by 

 which the inner coiled portion of a Scaphite, with outer deflected portion 

 broken away, can be distinguished from an Ammonite, unless it might belong 

 to a species known by its ornamentation, or some other specific characters. 



It is a very curious fact, noticed by d'Orbigny, that in unbroken speci- 

 mens of Scapkites, individuals of all sizes, from the smallest to the largest of 

 the known species, seem always to have the deflected portion, and the lip of 

 the aperture complete. Unless these smallest individuals are merely dwarfed 

 adult specimens, it certainly seems very difficult to account for this fact. 



The group Discoscaphites often has the characteristic deflection of the 

 body-portion so very slightly marked that both Dr. Morton and Dr. Owen 

 described the species as Ammonites. A careful examination of entire speci- 

 mens of this group, however, always reveals a more or less marked tendency 

 to develop this deflection of the body-whorl, even in such for-ms as S. Ckey- 

 ennensis, Owen (sp.), and S. Mandanensis, Morton (sp.), as may be seen by 

 turning to the figures on plate 35 ; while the nature of the septa corresponds 

 to that of Scapkites. 



On the other extreme, the Macroscaphites section very closely approxi- 



