GRAPTOLITES OF NEW YORK, PART 2 40I 



directed and occupies a large proportion of the theca; while the distal free 

 part is also, though less, oblique, the ventral wall of the theca exhibiting 

 but a slight sigmoid curvature. These forms still approach the Diplo- 

 graptus stage or rather — since both Diplograptus and Climacograptus 

 appear side by side with their typical characters in the zone with Phyllogr. 

 anna and Diplogr. dentatus, — the common ancestor of the two genera. 

 The preceding Axonolipa all possess plain, straight, tubular thecae, which 

 are inclined to the axis and not at all suggestive of the thecal form of 

 Climacograptus, with the sole exception of the Leptograptidae. The thecae 

 of the latter, as Lapworth, Elles and Wood have pointed out, possess a 

 slightly sigmoid curvature and have their apertures situated partly within 

 depressions (excavations). These curvatures and the apertural excavations 

 are in them developed very slightly indeed, but they may properly be con- 

 sidered as indicating the phylogenetic path to the thecal development of 

 Climacograptus, as well as of the Dicranograptidae. In forms of Climaco- 

 graptus, with zigzagged sutures, the sigmoid form of the thecae is more 

 distinct than in those with straight sutures, but also in the latter the steplike 

 form of the thecae is distinctly but a variation of the sigmoid curvature, 

 probably induced by the straightening of the rhabdosome. 



As will be observed in the descriptions of the species, there exist within 

 the confines of Climacograptus considerable differences in regard to the 

 relative lengths of the parallel and oblique parts of the thecae, the depths 

 of the apertural excavations and the directions of the free ventral walls and 

 apertures. On account of these various stages of development of the 

 thecal form it is sometimes difficult to assign a form properly to its genus 

 by the latter criterion alone and especially the distinction between Diplo- 

 graptus and Climacograptus becomes uncertain. This difficulty is still 

 increased by the fact that a typical Diplograptus may be thus compressed 

 and spl i t with the imbedding shale, that it assumes the aspect of a Climaco- 

 graptus and vice versa. This explains why some forms as Diplogr. 

 perexcavatus were not definitely placed in one of the two genera until 

 specimens in full or partial relief were obtained. 



