404 DESCRIPTIONS OF NEW SPECIES OF FOSSILS, 



fig. 4, 4, a, 5, 5, a, show an increasing degree of this development as the size of the 

 individual increases. In fig. 6 we have the parts fully developed, as shown in the 

 specimen, Plate V. fig. 1, a, and 1, b, which is 1.7 inches in its shortest diameter. In 

 all the smaller specimens the section is more broadly ovate, and in the minute ones 

 nearly circular, while the angle of the apex is nearly double that which we have 

 deduced from the measurement of parts of the adult specimens. 



We subsequently ascertained that a similar progression in the development of the 

 septa occurs in Ammonites and Scaphites from Nebraska ; and in even a more remark- 

 able degree than in the Baculites.* 



These facts in relation to Ammonites we have shown in the A. compJexiis, which is 

 represented in two stages of development ; while the septa of the inner volutions of 

 the small individual figured are no more undulated than some of the older and more 

 simple forms of Goniatites, leading unavoidably to the conclusion that the animal, in 

 its embryonic and extremely young state, is provided only with simple septa, like the 

 more ancient and recent Nautili. 



The Scaphites nodosiis of Owen, which, in its adult state, has extremely complicated 

 septa, has been proved, by a careful examination, to present in its successive stages of 

 growth the same development as occurs in Baculites and Ammonites, showing that 

 this law of development is common to these three genera ; from which we may infer 

 that it may pervade the entii'e family of Ammonitidse. 



These facts have an important bearing upon the study of this family of fossils, and 

 show in a remarkable degree how beautifully the progression from lower to higher 

 forms of animal organization, as exhibited in the introduction of successive creations 

 upon the same general plan from the older to the more recent geological epochs, is 

 here simulated and illustrated by the phases of development in a single individual 

 in its progress from the young state to maturity. 



• We are aware that the same characteristics have been shown in some European species of Am- 

 monites ; but these observations were made independently of any hints from other sources ; and we are not 

 able to ascertain from any publication within reach, that such features have been discovered in Baculites or 

 Scaphites. 



