Hyatt.] 254 [April 4, 



the outlines of their whorl in transverse section. They are in 

 succession, first, arcuate, then gyroceran, and lastly nautilian or 

 close coiled. 



In several series genetic lines of adult forms may be followed, 

 which lead by gradation from arcuate, cyrtoceran forms to close 

 coiled nautilian shells, the whole showing a connected series of 

 transitions in the form and outline of section, sutures, structure, 

 and position of siphon, and shell ornaments and apertures. In 

 some cases these graded series are in accord with the chronologi- 

 cal record, the straight appearing first, the arcuate either in com- 

 pany with them or later in time, and the gyroceran and nautilian 

 latest. 



We cannot of course claim that such perfect evidence has been 

 found even in the larger number of the following series. In some 

 of them, certainly, it is not an over statement to say that the 

 chronology of the evolution of form, the development of the 

 individual, the gradations in the adults, and the general differen- 

 tial characteristics all tell the same story, and are decisive for the 

 opinion, that in all the larger series of shell-bearing Cephalopoda 

 the nautilian shells belong to several distinct series and arose 

 independently from straight cones through the intermedium of a 

 graded series of arcuate and gyroceran or closely coiled forms. 

 The generic terms, Cyrtoceras, Gyroceras and Nautilus are really 

 only descriptive terms for the different stages in the development 

 of an individual, and also the different stages in the development 

 or evolution of the series of adult forms in time. In other words, 

 each of these genera as now used, include representatives of all 

 the different genetic series of Tetrabranchs, which are either 

 young shells in the corresponding stage of growth, or adult 

 shells in the corresponding stage of evolution. 



Our qualifications do not apply to the theoretical correlations, 

 which may be observed between the development of the individ- 

 ual in all its stages and the different forms of the group to which 

 it belongs ; these are very nearly perfect. It is impossible to 

 imagine the exact correspondence which exists between the 

 transformations of an individual during its growth and the differ- 

 ent adult forms of its own group, of the larva to the more ancient 

 forms, of the adolescent stages to the lower forms of its own 

 genus or family, except upon the theory of descent with modifi- 

 cations. 



