120 NEW YORK STATE MUSEUM 



Dicranograptidae on account of their sigmoid thecal form and the alternate 

 mode of growth of the thecae in the lower biserial portion of Dicrano- 

 graptus. The primitive straight form of the thecae of Diplograptus points 

 to Didymograptus and the sigmoid curvature of those of Climacograptus to 

 Leptograptus as the probable progenitors. Although the Leptograptidae 

 do not attain their acinic development until Trenton and later ages, they 

 appear early enough (Middle and Upper Skiddaw slates) to possibly have 

 furnished, in their earliest forms, the starting points of the phyla of the 

 Climacograptidae. 



Whether Diplograptus and Climacograptus also represent polyphyletic 

 groups such as Dichograptus, Tetragraptus and Didymograptus have been 

 found to be, instead of true genera [see Mem. 7, p. 5 53 ft J, can not be ascer- 

 tained until the phyla which apparently exist within them \scc descriptions] 

 have been traced backward into their ancestral " o-enera." The fact, how- 

 ever, that there exist not only such phyla within the two genera but also 

 very considerable differences in the form of the thecae would seem to 

 strongly suggest their polyphyletic character. These differences become 

 especially apparent in the genus Climacograptus where all stages from the 

 nearly straight Diplograptuslike thecae to highly sigmoidally curved ones 

 with slightly introverted apertures can be found. Such a form which seems 

 to stand between Diplograptus and Climacograptus is C . p u t i 1 1 u s which 

 on account of the very gentle curvature of its thecae has been currently 

 cited as Diplograptus p u t i 1 1 u s . 



Diplograptus d e n t a t u s of the third Deepkill zone is undoubt- 

 edly the first ancestor appearing in our rocks of the subsequently so power- 

 ful stock of D. foliaceus and its varieties and mutations, on one hand, 

 and D. a n g u s t i f o 1 i u s and D. euglyphus on the other. 



Climacograptus pungens, the first Climacograptid appearing 

 in the third Deepkill zone is distinctly a member of the same race with 

 C . p u t i 1 1 u s , C . t y p i c a 1 i s , mississippiensis, and u 1 r i c h i . 

 The close relationship of these forms is shown in their thecae, which are 

 furnished with mesial mucros and slightly exerted apertures, and in the 



