672 



XKW YORK RTATK MI'SEUM 





Fig. 63 Didymograptiis nitiilus Hull sp. 

 Proximal i>nit of iliabdbsome. Hcvei-se view. 

 I)i'e:v kill. x(i 



tance of 4 to 5 mm, then assuming a horizontal direction ; widening gradually 

 to a maximal Avidth of 2.4 mm (average but 1 . 6 mm), attaining each 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 slightly 

 concave, nonnal on axis of thecae (form- 

 ing angle of IBO" with axis of branch). 



Position and Jo('alitie!<. At the Deep kill this species has been found to 

 lie lestricted to gra[>tolite beds 1 and 2, which represent the Tetragraptus 

 zone. In tlie.se 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 



dent a t u s are represented, and the association with species from the latter 

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



associated with the major fauna, that of the 

 Tetragraptus zone. Strangely enough Ami 

 [1889, p.liek], Lapworth [1886, p.l84] and 

 Gurley [1896, p.295] do not cite this species 

 as having been foimd in the collections of 

 graptolites of the Quebec group, which they 

 investigated. T^apworth'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 which it is most similar. Matthew cites this species from 

 horizon 3d of the St John group in Ne\v Brunswick. In Great Britain the 

 iorm is widel}' distributed in the Middle Skiddaw slates of the Lake district. 



Fig. 67 Didymograptus nitidus Hall »p. idem. Shows 

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



V 



Fig. 68 Didymograptus nitidus 

 Hall ,vj). Distal part of branch. Deep kill. 

 x5.!!o 



