3jO NEW YORK STATE MUSEUM 



Dicranograptus spinifer Lapworth 



Plate 22; plate 23, figures 2, 3 



D i c r anogr ap t u s ramosus Hopkinson. Geol. Mag. 1870. 7:358, pi. 16, fig. 5 

 Dicranograptus ramosus Lapworth. Belfast Nat. Field Club. Rep't & Proc. 



Apx. 1877. v. i, pt 4, p.140; pi. 7, fig- 1 

 Dicranograptus spinifer Lapworth. Geol. Soc. Quar. Jour. 1882. 38:610 

 Dicranograptus ramosus var. spinifer (Lapworth ms) Elles &; Wood. Monogr. 



Brit. Grapt. pt 4 (Pal. Soc. 1904) p.176; pi. 24, fig. 8a-c 



Elles and Wood describe two varieties of D. ramosus, viz, var. 

 spinifer and var. longicaulis. The former of these is in certain 

 outcrops of the Normanskill shales, the common representative of D . 

 ramosus, while the latter has not yet been observed here. It is stated 

 of the var. spinifer, that Lapworth already recognized it in 1877 and 

 that it is characterized " by (a) the fusiform shape of the biserial portion 

 which is also considerably longer than that of the typical form, and (/>) by 

 its strongly spinose thecae." These characters are also well shown in 

 our specimens [see plate 22]. The length of the biserial section varies 

 considerably, viz, from 8 mm to 27 mm, the width is .5 mm at the sicular 

 extremity and may rise within 10 mm to 2 mm, to decrease again slightly 

 towards the bifurcation. The final angle formed by the uniserial branches 

 is undoubtedly a little larger than in the typical ramosus and averages 

 40 , but may rise to 45 . On the other hand the uniserial brandies of most 

 specimens are markedly narrower than those of the typical form, and falling 

 below 1 mm, while at the same time the thecae are more widely arranged 

 (but 8-9 in 10 mm, see figure 263) and provided with rather long excava- 

 tions. The mesial spines do not seem to extend in any of the specimens to 

 more than a dozen thecae on either side, independent!)' of the number of 

 thecae in the biserial section (which may rise to 25 on either side in our 

 material) ; while of the British specimens it is stated that " All the thecae 

 of the biserial portion (with the possible exception of sonic ol the distal 

 ones) are provided with long and stout mesial spines." The New \ ork 

 form is, hence, again varying from the British variety in some of its 

 features. 



