3 2 4 



NEW YORK STATE MUSEUM 



branches diverge nearly as much as in the types of parvangulus, but 

 they still retain their torsion [see pi. 21, fig. 5]. This fact would also 

 refute the possible assumption that oblique compression of specimens of 

 parvangulus could produce the parallelism and smooth aspect of the 

 branches, seen in diapason. 



It is probable that this torsion sets in already at the sicular end of the 

 biserial portion, for a close scrutiny of the same will always show that if on 

 one side the free parts of the thecae are convex and shown in their full 

 profile view they are concave at the opposite side or seen there slightly 

 obliquely and that this condition is reversed farther up [see fig. 252, 253]. 

 We infer from these facts that in this variety a very gradual torsion of the 

 branches takes place and that this is combined with a spiral growth of the 

 branches as in D. furcatus; that hence the now parallel branches if 

 found long enough would be seen to form a very long 8 and we have indeed 

 seen a specimen in which the extremities of the branches come again in 

 contact. D. nicholsoni var. diapason further distinguishes itself 

 from its associates on the slabs by its strongly glossy appearance, indicating 

 a greater thickness of the periderm than found in most other species. It is 

 probable that this is causally connected with the torsion of the branches. 



This interesting variety is known to us only from the Normanskill 

 shales at Stockport and Glenmont, N. Y. 



Dicranograptus nicholsoni Hopkinson var. whitianus Miller 



G 1 a p t o 1 i t h u s (C 1 i m acograpt u s) r a m u 1 11 s White. Exped. & Sur. West 100th 



Merid. Prelim. Rep't. Invert. Foss. 1874. p. 13 

 Graptolithus (Climacograptus) ramulus White. Wheeler Sur. West 100th 



Merid. Rep't. 1875. v. 4, pt 1, p. 62 ; pi. 4, fig. 3a-c 

 Graptolithus whitianus Miller. Cat. Am. Pal. Foss. Ed. 2. 1S83. p. 269 

 Dicranograptus ramulus Herrmann. Nyt Mag. f. Naturvidensk. 1886. v. 29 

 Dicranograptus nicholsoni var. whitianus Gurley. Jour. Geol. 1896. 



4:72, 300 



The type of this form, which was obtained 5 miles north of Bel- 

 mont, Nev., is in the National Museum. Dr Gurley has made the subjoined 



