GRAPTOLITES OF NEW YORK, PART 2 205 



the doubtful genera of graptolites, since no thecae or apertures on the 



branches have been observed. The fact that forms properly referable to 



this genus were rarely found, aided greatly to keep it in mystery. The only 



one who advanced our knowledge of the genus was Lapworth, and his 



observation that Rastrites barrandii Hall is a branch of Thamno- 



graptus added another peculiar character, that of a one sided branching, to 



the curious group. The original diagnosis of the genus is : 



Bodies consisting of straight or flexuous stipes (simple or conjoined at 

 base?) with alternating and widely diverging branches; branches long, 

 simple or ramose in the same manner as the stipe. Substance fibrous or 

 striate ; the main stipe and branches marked by a longitudinal central 

 depressed line, indicating the axis. Cellules or serratures unknown. 



The rather large collections which we have gathered from various out- 

 crops of the Normanskill shale, have furnished us material that not only 

 fully verifies Lapworth's observation of the identity of Rastrites bar- 

 randii with Thamnograptus, but also demonstrates the identity of T. 

 typus and T. capillar is, the two Normanskill species distinguished 

 by Hall [see postca p. 209]. Thereby the complicated system of branching 

 of a Thamnograptus becomes known. 



T . t y pus and T. capillaris are shown here [see p. 208] to be the 

 main stipe with its stripped-off branches of the first order, the former repre- 

 senting the proximal portion of the rhabdosome, the latter the distal one, 

 while the branches of the higher orders are represented by Rastrites 

 barrandii. Furthermore, sections of T. typus and specimens of 

 "Rastrites barrandii" have been observed, which exhibit a distinct 

 thecal composition [see pi. 12, fig. 9, 13, 14]. It is seen in these that the 

 thick stipe of T . typus must have originated from a series of slender 

 tubular thecae which have become enveloped by thick secondary conchio- 

 linous layers and that the " Rastrites" form consists likewise of a series of 

 slender tubular, slightly curved thecae with but little projecting apertures 

 which all lie on one side of the branch, and from the lower margins of which 

 again very long, hairlike branches proceed that as a rule are broken off at a 

 short, uniform distance. Whether these again are composed of thecae has 



