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NEW YORK STATE MUSEUM 



Fig'. 63 Didymogi'ap tus nitidus Hall $p. 

 Proximal part of rhabdosomc. Reverse view. 

 l)eei)l;ill. xG 



tance of 4 to 5 mm, tlien assuming a liorizontal direction ; widening gradually 

 to a maximal width of 2.4 mm (average but 1 . 6 mm), attaining eacli a 

 maximal length of 124 mm. Thecae closely arranged (11 to 13 in 10 mm), 



slightly curved, inclined to the axis at an 

 angle of about 40° ; three times as long 

 as wide, in contact for two thirds or less 

 in the proximal and for three fourths of 

 their length in the mature portion of 

 the branch. Apertural margin slight]}' 

 concave, normal on axis of thecae (form- 

 ing angle of 180" with axis of branch), 



Position, and localities. At the Deep kill this species has been found to 

 1)6 restricted to graptolite beds 1 and 2, which represent the Tetragraptus 

 zone. Ill tliese it is quite com- 

 mon. Hall reports it from the 

 shales of the Quel)ec group at 

 Point Levis. As at the latter 

 point the Tetragraptus zone and 

 the zone with D i p 1 o g r a p t u s 



d e n t a t u s are repi'esented, and the association with species from the latter 

 zone is always carefully mentioned, it is to be inferred that Hall's types were 



associated -with the majoi- fauna, that of the 

 Tetragraptus zone. Strangely enough Ami 

 [1889, p.liek]. Lap worth [1886, p.l84] and 

 Grurley [1896, p.295] do not cite this species 

 as having been found in the collections of 

 graptolites of the Quebec group, which they 

 La])\v()rth's material did not contain representatives of the 

 Tetragraptus zone; and in Ami's lists it has most probably been referred to 

 D. e X t e n s u s, to \\'hich it is most similar. Matthew cites this species from 

 horizon 3d of the St John group in New Brunswick. In Great Britain the 

 form is Avidely distributed in the Middle Skidda^' slates of the Lake district. 



Fig. 67 Didymograptus nitidus Hall sp. idem. Shows 

 point of origin ot first theca and growth lines. Deep kill. x5.25 



Fig. 68 Didymograptus nitidus 

 Hall .sp. Distal part of branch. Deep kill. 

 x5.<J5 



investigated 



