390 UNITED STATES GEOLOGICAL SURVEY OF TLIE TERRITORIES. 



These modifications arc of eour.se improvements, and have been adopted 

 by several later authors, particularly the term siphonal lobe, which I have 

 found convenient; and, as it is readily understood and applicable to shells of 

 all forms, while it avoids the use of either of the terms dorsal or ventral, 

 it ought to be generally adopted. The other name, however, used by Pictel 

 for the antisiphonal lobe — -tliat is, his name median internal or dorsal lobe — 

 seems to me to be objectionable, because it is desirable to use terms that 

 will be equally applicable to any and all of the various types of this great 

 group of shells, including many genera of different families, and presenting a 

 great variety of forms, from the straight Baculites. to involute and spiral types. 

 It would certainly be a misnomer, and cause confusion in the minds of stu- 

 dents, to continue to call the little lobe directly on the opposite side from the 

 siphon, in Baculites, the median internal or dorsal lobe, when none of the 

 lobes in a straight shell of this kind can be properly called internal. Yet the 

 lobe alluded to in the genus Baculites corresponds exactly to that which Pictet 

 calls the median internal in the involute and curved genera. 



For these reasons, I have preferred to adopt the name antisiphonal lobe 

 for that directly opposite the siphonal, in whatever type it may be met with, 

 whether it be a Baculites, an Ancylooeras, or an Amnronitoid shell. This term 

 has already been used by some European palaeontologists, and will always be 

 readily understood without explanation, while it avoids any perplexing allu- 

 sion to either dorsal or ventral side. 



I have also long thought that von Buch's nomenclature might be much 

 simplified by a still further modification; that is, by discarding his names 

 dorsal saddle, superior Intend lobe, lateral saddle, 'inferior lateral lobe, ventral 

 saddle, and auxiliary lobes and saddles, and simply designating the whole of 

 these flexures, whether \'r\v or many, on each side of the siphonal lube, as 

 lateral lobes and sinuses, numbering each series consecutively from without 

 inward to the umbilical margin. This avoids all circumlocution, and enables 

 one to refer directly to any particular lobe or sinus, simply, for instance, as 

 the fourth lateral lobe, or the third lateral sinus, in descriptions or com- 

 parisons. 



By reference to the cut of a septum of I'/acenticeras placenta, given in 

 connection with that species farther on. the terminology that I have used in 

 the following descriptions will be more clearly understood. 



The student should also be informed that, in comparing the septa of 

 these shells for the purpose of identifying species, genera, or larger groups, it 



