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epinepionic periods of development in obedience to the law of the 

 cycle does not carry the structure back with it to a repetition of the 

 orthoceran siphuncle and sutures. 



The structure of an individual during its development might be 

 represented graphically by an irregular spiral of one incomplete 

 revolution which describes a curve, continually increasing its dis- 

 tance from the point of departure until the meridian of the ephebic 

 stage is reached, and then beginning to return. Such a curve would 

 always as a spiral rise more or less vertically, and consequently, 

 even if it completed the revolution, must terminate in space. It 

 might, perhaps, reach nearly to the same imaginary vertical plane, 

 but never to any point approximate to that of its departure. Structure 

 separates the extremes of life as widely as possible, and does not 

 permit us to regard them as approximate, nor can one regard old 

 age, however complete its return in external form, as a reversion. 



One of the most noteworthy contributions of bioplastology is 

 that it gives proper values to this class of analogies and shows them 

 to be constantly recurring in the individual and in the phylum in 

 obedience to well-ascertained laws of morphogenesis. 



The different stages have been described by Dr. Beecher among 

 Brachiopoda, Dr. Jackson among Pelecypoda, and the author 

 among Cephalopoda; and Buckman and Bather and also Blake* in 

 England, and Wiirtenberger in Germany have admitted their exist- 

 ence, and the last redescribed them. Wiirtenberger has admirably 

 described the phenomena of bioplastology as they occur among 

 Ammonitinae, and correctly interpreted the law of tachygenesis and 

 its action in these forms, but failed to quote either Prof. Cope or 

 the author. This omission was not so remarkable as the fact that 

 Neumayr and some other investigators, after they had received the 

 printed records of the work done in the same direction in this 

 country, continued to quote Wiirtenberger as the sole discoverer of 

 these phenomena and of the law of tachygenesis. Wiirtenberger's 

 work was apparently independent, and it has higher value on that 

 account, but it needs rectification from a historical point of view. 



Buckman and Bather propose to use the prefix " phyl " for forms 

 occurring in the phylum which represent in their adult characters 

 stages in the evolution of the phylum corresponding with those in 

 the development of the ontogeny, and give an instructive table in 



*" Evolution and Classification of Cephalopoda, " Proc. Geo!. Assoc. Land., Vol. xii, pp. 

 276-295, 1892. 



