GRAPTOLITES OF NEW YORK, PART 2 255 



Deepkill beds, than of any other form and a comparison of their dimensions 

 furnishes proof of their phylogenetic relationship. 



For this reason, I have also referred this species to Didymograptus 

 instead of to Leptograptus, as Lapworth has clone. Hall's types do not 

 show any of the characters either of the proximal part (two crossing canals) 

 or of the thecal form (sigmoid curvature and introverted aperture), cited 

 as diagnostic for the genus Leptograptus in the Monograph of British 

 Graptolitcs. On the other hand, while D. sub tenuis is by no means 

 common in any of the localities, there is found in the same beds in much 

 greater frequency a mutation of Leptograptus flaccidus ( here 

 described as L. flaccidus mut. trentonensis). The latter, from 

 its general form and by comparison with Hall's figures — which, where the 

 types are not at hand, alone are available for identification — -might easily 

 be taken to represent D. subtenuis. Lapworth has inserted in his 

 manuscript report on the Stockport graptolites a full description of "Le p- 

 tograptus tenuis," that completely tallies with the latter mutation, and 

 since it is added, that L. tenuis is nearest related to L. flaccidus, 

 there is little doubt that these two similar forms of our Normanskill shale 

 have not been separated. 



While fragments of the mature branches of D. subtenuis and 

 Leptograptus flaccidus mut. trentonensis are not always 

 distinguished with ease, the initial parts of their rhabdosomes are widely 

 different \comp. fig. 160 and 172]. On the other hand, the proximal portion 

 of the rhabdosome in our species is very similar in aspect and dimensions 

 to that of Nemagraptus exilis var. linearis, but a comparison 

 of the siculae and the form of the thecae will readily show their distinctness. 



Didymograptus ? eleg-ans (Emmons) 

 Monograpsus ? elegans Emmons. Am. Geol. v. 1, pt 2. 1856. p. 106-7, pi. 1, 



fig. 27 



Original description. Outer edge of the serrations straight and nearly 

 parallel with the opposite edge ; depth of the serration equals one half of 

 the width of the stipe. Figure 14 enlarged. The width of the stipe is 



