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Thus, although both Barrande and Hall have courteously ac- 

 knowledged each other, and have mutually joined names as au- 

 thority for this genus calling it Trochoceras, Barrande and Hall, 

 the Trochoceras of the former is not the same as that of the latter, 

 and the name of one or the other or both must be dropped. 



I have therefore retained Trochoceras Barrande, and propose for 

 Hall's remarkable forms, Trochoceras Gebhardi and turbinatum, 

 the new name of Mitroceras* with Mitroceras (Troch.) Gebhardi, 

 sp. Hall, PL lxxvii and lxxviitf as the type. 



It must not be supposed that all forms of Nautiloids having the 

 turbinate spiral are devoid of impressed zones* There are some 

 species that do have this characteristic, but it is invariably slight, 

 and occupies necessarily a position on the sides rather than on the 

 dorsum of the whorls. 



Lituitidoz. 



Recent investigations have shown that this group, instead of in- 

 cluding about all of the unrolled, shell-covered Cephalopoda of the 

 Paleozoic, must be limited to certain well-defined homogeneous 

 series with peculiar characteristics. 



My observations lead me to think that Lituites is a degenerate 

 form of Cyclolituites, a view similar to that of Holm and Schroder, 

 who regard this genus as the radical of the Lituitidse corresponding 

 to the younger stages of true Lituites. 



The genera included in this family form a degenerating series 

 which may have evolved from Cyclolituites, or some form that this 

 most closely represents, becoming specialized by reduction of the 

 spiral and simplification or loss of correlative characters during 

 growth of the whorl, lessening curvature of the annuli and lines of 

 growth and in the outlines of the apertures, until finally, in the ex- 

 treme forms of Rhynchorthoceras, the whole shell is straight or 

 orthoceran, except during the earliest stage, the nepionic, and in 

 that it is not a perfect coil. 



This process takes place through the disappearance in the earlier 

 stages of the progressive characters of Cyclolituites and the gradual 

 replacement of these by characteristics that first appear in the 

 paragerontic stages of such species as Ang. prcBCurrens. That is to 

 say, Rhynchorthoceras has from a comparatively early stage the ven- 



*Frorn Mir pa, a girdle, but also used for " turban," in which sense it is here quoted. 



