GRAPTOLITES OF NEW YORK, PART 1 691 



Ptilograptus tenuissimus sp. nov.. 



Plate t flarure 18 



There has been found in graptolite bed 3 of the Deep kill section 

 an extremely delicate rhabdosome, in fact, so tenuous that, notwithstand- 

 ing its considerable length, it is only with difficulty visible to the naked 

 eye. 



Descriptiov: The specimen consists of a single principal stem or branch 

 which is 28.6 mm long, but not more than . 1 ram wide in its thickest part 

 and very gradually tapering toward the distal end. The branchlets are 

 arranged bipinnately, extremely thin (not more than . 03 mm thick near their 

 base), filiform, reaching a length of 8 . 2 mm. Those of one side are about 

 2 . 2 mm apart and diverge at an angle of about 40° from the stem. No 

 traces of thecae or thecal apertures are noticeable anywhere on the 

 branches. 



Position and localities. Graptolite bed 3 (zone with D i d y m o - 

 graptus bifidus) at the Deep kill. 



Remarks. This form is easily distinguished from P. plumosusby 

 the much longer and slenderer stem and branchlets and the greater distance 

 between the bases of the latter, which is about four times as great as in 

 the specimen of P. plumosus figured here and twice as great as in 

 the type specimen of that species, which, extending to a more proximal 

 part of the rhabdosome, has the branches a little farther apart. 



This species bears some similarity to the form described by Hall as 



Thamnograptus anna from the same horizon, but differs by its 



straight, not zigzagged principal bi-anch and the greater inclination of the 



branchlets. 



DicTYONEMA Hall. 1852 



The generic term Dictyonema was proposed by Hall in Palaeontology 

 of New Yorh, volume 2, for a group of fossils which hitherto had been 

 referred to such widely different groups as the gorgonias, bryozoans and the 

 algae. It is to Hall's credit to have first observed the thecae and clearly 



