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regarded as an actual characteristic belonging to the shell. I now, 

 however, consider it probable that this fragment, in spite of its 

 great similarity to the first volution of Phuronautilus superbus, 

 should not be placed with this species, nor regarded as a first 

 whorl, but as another new species. I incline much more at present 

 to the view that the impression (the author's dorsal furrow) upon 

 the concave side might have originated from a whorl now broken 

 away. ' ' 



NautilidcR. 



Without attempting at present to limit the chronologic distribu- 

 tion of this family, it is necessary in this connection to make some 

 remarks with reference to my observations on the general affinities 

 of the genera described in this paper, which are all Mesozoic. 



Digonioceras is obviously the most primitive type yet found in 

 the Mesozoic, and the most primitive or most generalized species is 

 Digonioceras excavahim, as figured by D'Orbigny. The broad first 

 whorl of this species is persistent in adults and so also is the slight 

 amount of the involution and the discoidal character of the coil. 

 The digonal and approximately nephritic outline of the young in 

 the paranepionic is succeeded by a subtrigonal outline in the adult. 



This is substantially paralleled by the development of the species 

 of Cenoceras, Cymatoceras, Eutrephoceras and Nautilus. 



All of these are apt in their nepionic substages to bring out the 

 nephritic, and in the paranepionic the subtrigonal form of whorl 

 with a broad dorsum, converging lateral zones and more or less 

 subacute or elevated venter. This occurs even when the nephritic 

 outline or some other is assumed in the later stages. 



There is, therefore, in all of these genera some direct reference 

 to the form of the ephebic stage of Digonioceras. 



This fact is of great importance in connection with the assump- 

 tion made in this memoir, that after the Trias the survivors of the 

 Nautiloids are all nautilian shells and bear the marks of their descent 

 from close- coiled ancestors and are not directly connectible with 

 straight or arcuate types as the nautilian shells of the Paleozoic 

 often are. 



Digonioceras, n. 



Digonioceras excavatum, was described in my Genera of Fossil 

 Cephalopods, as a member of the genus Endolobus surviving in the 



