550 NEW YORK STATE MUSEUM 



The connection between tlie Axonolipa and Axonophora is still obscure. 

 The fact is that, where in the Lower Siluric the axonophorous genera 

 appear — in the Mt Moreno beds and the Deep kill section in the zone with 

 Diplograptus dentatus, with the genera Diplograptus, Climacograp- 

 tus, Glossograptus, Trigonograptns and Retiolites — they at once come on the 

 field in great force, and that the Axonolipa then rapidly disappear. Further, 

 no form is known in the preceding zones which could be conclusively held to 

 announce or foreshadow this new departure in graptolitic structure. We 

 have here, hence, apparently an interesting illustration of the principle empha- 

 sized by Hyatt : that types are evolved more quickly near the point of origin, 

 and that there are greater structural differences between genetic groups of the 

 same stock while still near this point than appear subsequently. 



The genera Diplograptus, Climacograptus, Trigonograptus and Eetiolites, 

 appearing suddenly side by side, encompass already the full amplitude of the 

 structural differences as expressed in the position of the thecae found among 

 the diprionid Axonophora ; while Retiolites already indicates the perforation 

 of the peridermal wall, that becomes fully developed in later phases, and 

 Glossograptus already presents the extreme growth of spinous appendages in 

 this class of graptolites. 



It is known that a strong tendency to the perfection of the structure 

 attained by the Diplograptidae and which insures a stable upward growth of 

 the thecae, has manifested itself distinctly in the preceding Dichograptidae 

 by the assumption of a reclined position of the branches and led to the 

 development of such forms as the Phyllograptidae, the reclined Tetragrap- 

 tidae (Tetragraptus similis) and Didymograptidae, as D i d y m o - 

 graptus caduceus (gibberulus). It would hence seem proper to 

 look among these reclined forms for the ancestors of the Diplograptidae. 



An interesting observation, probably suggestive of the path of derivation 

 of Diplograptus from the Dichograptidae, has been recorded by Tornquist 

 [1901, p.23] and verified by Elles and Wood [1901, p.53]. The latter 

 authors state of Didymograptus gibberulus: " The crossing canal 

 is clearly seen just below the apex of the sicula, and rather above the initial 



