PARALLELISM. 185 



etically, i. e. with sole regard to the evolution of dis- 

 tinct series which may be traced from their origin to 

 their termination in time. 



"Of these last there are some in every period trace- 

 able with more or less completeness by gradations of 

 adults back to orthoceran or cyrtoceran ancestors. Of 

 these series of adults, some pass through only the or- 

 thoceran and cyrtoceran modifications, others have 

 the orthoceran, cyrtoceran, gyroceran, and nautilian, 

 but those having the latter and the nautilian-involute 

 are of extreme rarity until the Carboniferous is reached. 

 After this the nautilian shells begin to predominate in 

 every series, ultimately becoming the sole representa- 

 tives of genetic series. 



"Such series are, of course, frequently so closely 

 parallel that it is possible to follow them, and show 

 they are distinct only by means of certain genetic char- 

 acters, the apertures, the structure of the siphuncle, 

 the sutures and septa, and sometimes, although very 

 rarely, all of these internal characters may show dif- 

 ferences peculiar to some one genetic series in which 

 the regular gamut of forms is passed through in the 

 usual succession. Neglect of the comparative study 

 of the stages of development and decline, and of the 

 obvious parallelisms between these and adults of an- 

 cestral forms, have caused naturalists, notably Bar- 

 rande, to make artificial classifications in which about 

 all straight forms, with the exception of some in which 

 the siphuncles were notably distinct to be classed as 

 Orthoceras, most of second kind as Cyrtoceras, most 

 of the third kind as Gyroceras, most of the fourth and 

 fifth kinds as Nautilus. 



"To such authors the involute-nautilian forms of 

 the Silurian and the existing fauna were considered to 



