33^ 



NEW YORK STATE MUSEUM 



Or-z- 



*sp<-j- 



from the Shakopee formation in Minnesota is of special interest in 

 relation to the genetic history of this genus and in our opinion stands 

 at the opposite end of the series from P. explanator. While 

 in the latter the ectosiphonal wall distinctly consists of the 

 coalesced reiiexed margins of the septa (septal necks), Clarke's 

 careful description and figures [see fig.25] demonstrate that in 

 P. newton-winchelli the funnels or septal necks are 

 only very short and the siphuncular wall is distinctly formed by 



a secondary formation, " the annuli ".^ 

 If we adopt ^yatt's fundamental divi- 

 sion of the Nautiloidea, we find the 

 genus Piloceras brought under the Holo- 

 choanites which are characterized by the 

 extension of the funnels from one sep- 

 tum to the next preceding or beyond. 

 Piloceras newton-winchelli 

 is hence not a member of the genus 

 Piloceras as defined by Hyatt, indeed 

 it has the ectosiphuncular structure of 

 another suborder, the Orthochoanites ; 

 or has advanced in the character of its 

 ectosiphuncle from the Cameroceras 

 stage found in the other Piloceras forms, to the later Orthoceras 

 stage. The relation of this form to the typical Piloceras appears to 

 us identical with that of Endoceras burchardii Dewitz 

 to the true Endoceras, the latter being a species which, while retain- 

 ing the habit of an Endoceras has, as Holm has shown [1897, p. 171] 

 the ectosiphuncular structure of an Orthoceras. Holm proposed the 

 genus Baltoceras for this form, a genus which is considered by 

 Hyatt as the first and most primitive of the genera of Orthoceratidae. 



M^S 



:>e^ 



^^S- 25 Clarkoceras new- 

 tt)n-winchelli Clarke (sp.). En- 

 largement of portion of section to 

 show the siphuncle [S] ; endosipho- 

 sheaihs [ss]; ectosiphuncle [wsJ; 

 endu>iphotube [es] ; septa [s/] and 

 annuli la], (Copy from Clarke) 



ift is doubtful whether these annuli or siphuncular segments of the Ortho- 

 choanites form a homologue to the continuous " endosipholining " of Camero- 

 ceras, as it would appear at first glance. The endosipholining is considered 

 by Hyatt as composed of the upper unresorbed ends of the endosiphosheaths, 

 while the siphuncular segments find their fullest development where, on 

 account of the reduction of the siphuncle, no more endosiphosheaths are 

 formed. Nor is any genetic connection between the segments and the 

 endosiphosheaths apparent in text figure 25. 



