GRAPTOLITES OF NEW YORK, PART 2 



into- parallelism, or even approximates them still further. Thecae forming 

 bent tubes with the tip introverted, 24 or 25 in 25 mm ; some of them (prob- 

 ably all on basal portion ; material here uncertain) bearing an acute spine. 



This variety is a well marked one. In form the most characteristic 

 specimens approach closely D . z i c z ac minimus Lapw., but our form 

 is at least twice as large as Lapworth's, and besides D. ziczac appears 

 not to occur in our strata. From the characteristic caliper-shape as a basis, 

 the variety shades into var. parvangulus, which has the branches 

 straighter and continually divergent. 



Horizon and locality. Lower Dicellograptus zone, Stockport, N. Y. 



Remarks. Since Dr Gurley did not publish any figures of his new 

 variety, we insert here drawings of the type specimens [text fig. 249-51], 

 made under his supervision. The collection obtained by the State Museum 

 in the railroad cut at Glenmont contains a finely preserved series of speci- 

 mens of diapason; they are associated with Dicellogr. gurleyi, 

 Climacogr. parvus and Glossogr. ciliatus, but not with D. 

 n i c h o 1 s o n i var. parvangulus. They permit the elucidation of some 

 additional characters of the variety. 



The thecae possess strongly convex ventral walls in their free portions ; 

 their apertural parts are strongly introverted and introtorted. The septum 

 begins after the second pair of alternating thecae. The biserial portion 

 has the characters of D . n i c h o 1 s o n i and as Gurley surmised the 

 mesial spines extend to the bifurcation [see fig. 253, 254]. Besides the paral- 

 lelism of the uniserial branches, their most striking character is their 

 ribbon-shaped smooth form, with straight margins, a feature well shown in 

 Gurley's drawings. Our camera enlargements give the explanation to this 

 character by which diapason can at once be distinguished from all other 

 forms of D . n i c h o 1 s o n i . When the test is preserved, as in figure 252, 

 one branch will give a frontal, the other a dorsal view ; where one branch 

 is preserved with the test, the other as impression, as in figures 253 and 254, 

 both will give like views. The biserial portion at the same time gives 

 always a profile view of the thecae. It is, therefore, evident that at the 

 bases of the uniserial branches a torsion of the rhabdosome has taken place. 

 This feature is probably more important and characteristic of the variety 

 than the parallelism of the branches, for we have specimens in which the 



