670 NEW YORK STATE MUSEUM 



Westrogothia. It is not reported from the Bohemian. Belgian and 

 French graptolite beds. McCoy identified a graptolite, which he found 

 abundantly in Victoria, with this species, but his description and figures indi- 

 cate a somewhat closer arrangement and a greater inclination of the thecae. 

 In Europe the form is restricted to the lower Phyllograptus shale, corre- 



_ sponding to the Tetragraptus zone 



of the Deep kill, in which it 

 /' also in America finds its principal 



development; but, according to 



Fig. 64 Didy mograptus extensus Hall sp. Frag- Tjflnworth if, nflS!s;p's Iiptp nlssn 

 ment of mature part of branch. Deep kill, xo.25 -Lia,p\\Oim, It pdbSeb Ueie aiSO 



into the next higher zone with 

 D. bifidus and Phyllograptus anna. 



Memarlcs. D. extensus can, in the material from the Deep kill, be 

 readily distinguished from its congeners by the great length and nearly uni- 

 form width of the branches and its relatively broad thecae. In this repre- 

 sentative the genus Didymograptus attained here its maximal development ; 

 for individuals like that figured on plate 14 [fig.l] may have reached 1 m in 

 length. Also in final width the Deep kill examples of the species pass con- 

 siderably beyond the maxima observed elsewhere. 



Intermingled with typical specimens of D. extensus occur numerous 

 others which have the appearance of those figured on plate 13 [fig.l7, 18]. 

 They are as a rule more flexuous, have a uniform 



width (about 1.9 mm) throughout their length, and ^ >il^p^l^^^^3l^ 

 possess a constrictiform appearance of the thecae. 



Fig. 65 Constrictiform appear- 



As these are the principal characters of Hall's |.?^^tuf. ''Deep"km.°x2^''^^'"°" 

 species D. c o n s t r i c t u s, we have no doubt that 



the latter species is based on specimens which became compressed in a 

 certain direction, namely obliquely dorsolaterally instead of laterally, so 

 that the frontal or apertural side of the thecae is partly exhibited. On this 

 account Hall found also that the " common body occupies a very small pro- 

 portion of the entire width, and its limits are not distinctly defined." The com- 

 mon canal lies in this mode of preservation, behind the exposed side. The 



