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tions, making a complete series of all of the substages of develop- 

 ment in Nautilus pompilius and more or less of other species, and 

 also to Dr. R. T. Jackson, for similar material. I hope to use this 

 material more extensively and effectually in the future. In this 

 paper full justice cannot be done to the work of Dr. Beecher or 

 Mr. Brooks. 



This generic name, heretofore supposed to include nearly all 

 of the coiled or nautilian forms of Nautiloidea and still used by 

 some conservative paleontologists in this way, is really not applica- 

 ble to any forms except the living species of nautiloids and possi- 

 bly some shells in the Tertiary. Even these last cannot be satisfac- 

 torily referred to the genus Nautilus until their nepionic substages 

 have been worked out. 



The genus Eutrephoceras is a near ally but still distinct in most 

 of its characteristics. The broad outline of all of the epinepionic 

 stages of growth, the general position of the siphuncle, dorsad of 

 the centre, and the distinct sutures of Eutrephoceras separate the 

 species. The minute umbilical perforations and closer coiling of 

 the younger substages of the conch in Eutrephoceras show also that 

 it is the terminal group of some other genetic series than that to 

 which Nautilus probably belongs. 



The genus Cymatoceras of the Cretaceous differs in the broad 

 costations as well as in the outline of the nepionic whorl. The 

 sutures of this genus are more like those of Nautilus than the 

 sutures of Eutrephoceras. 



The genus Nautilus is obviously still more remote from Cenoce- 

 ras of the Jura in the sutures of all stages and form of the ananepi- 

 onic and succeeding nepionic substages, although in the outline of 

 the ephebic whorl and position of the siphuncle there is close 

 approximation. If one excepts the comparison of the ananepionic 

 substage, which is obviously similar to that of Eutrephoceras Dekayi, 

 being only more compressed, the nepionic stage and the ananeanic 

 substage are very close in aspect to those of Digonioceras, although 

 the succeeding substages become quite distinct. 



I cannot in this memoir give full descriptions of the substages of 

 development which I hope to treat fully in the future. It will suf- 

 fice to refer to the accurate drawings of Mr. Brooks, given on PI. i, 

 and to notice the fact that young shells and preparations now in 

 my possession of Nautilus umbilicatus, pompilius and macromphalus 

 show no variations in their characteristics worth noticing here. It 



