GRAPTOLITES OF NEW YOKK, I'AUT 1 GOl 



The different directiou of compression alters the aspect of tliis species 

 very considerably. The distinctive characters cited before siifKce however 

 fully for its recognition in all cases. We have figured some of the more strik- 

 ing aspects. It appears that, on the whole, the l:)rauches of the Deep kill 

 specimens are somewhat narrower, or at least do not reach the maximal width 

 cited by Hall and EUes, while in all other respects, specially the length Avhich 

 they attain, they do not differ from the descriptions given by these authors. 



In a paper on the growth and development of G o ii i o g r a p t u s 

 t h n r e a u i , the writer figured two extremely minute, apparently young 

 specimens of a Tetragraptus, which he then thought 

 referable to this species. Subsequent investigation has 

 shown, that notwithstanding the resemblance between 

 these and the mature specimens of T. similis, the 

 young of the latter form are quite different, their sic- 

 nlae beina: lare-er and stouter, and not proiectinc below Fig. ei Tetragraptus 



° ° i d ^ similis Hall s)j. Frag- 



the first few thecae. As these minute rhabdosomes could to^^how*^^ thTcharalj'tw'iftlu 



-, p -• 1 • i> m 1 aspect of the thecae iu the 



not be referred to any other species oi i etragraptus, they compressed condition. Deep 

 have been described as a new^ type ( T . p y g m a e u s ) . 



Holm succeeded in isolating specimens of T. similis (bigsbyi) 

 and thus elucidating the structure of the proximal parts. He also gives 

 figures of a very young specimen, showing the " Didymograptus stage," i. e. 

 the sicula, the left and right thecae and the connecting canal. Some of his 

 instructive figures have been copied in the discussion of the genus. 



The surfaces of graptolite bed 2 at the Deep kill are covered with 

 growth stages of this species. These verify Holm's observations, so far as 

 their flattened condition permits the study of their original structure. We 

 figure several of these stages which exhibit important features [pl.l2, fig.8-10]. 

 Specially remarkable among them is the very long, extremely thin filamentary 

 uema, which in one specimen [fig.lOj is seen to end in a relatively large, 

 thinly chitinous disk with a somewhat thickened central part. 



Very fi'equently the two branches of one side alone are preserved 

 [fig.58], whereby the fossil receives the appearance of a Didymograptus, of 



