GRAPTOLITES OF NEW YORK, PART 2 93 



formed habitually while the rhabdosome was still a part of the synrhabdo- 

 some. In others, however, as Climacograptus parvus, one may 

 observe great numbers of associated rhabdosomes without any trace of 

 vesicles, while in another layer the vesicles may be very frequent. 



The bed at Glenmont, which contains them in such great numbers, is 

 remarkable for the association of this Climacograptus with an equally great 

 number of rhabdosomes of Lasiograptus mucronatus. The latter 

 is a notably light form on account of its very thin peridermal walls ; and its 

 association with the apparently thick walled rhabdosomes of Climaco- 

 graptus parvus indicates that the weight of the latter had been bal- 

 anced to such an extent that they could drift and settle at the same rate 

 as those of the Lasiograptus. The frequent absence of vesicles in long- 

 stemmed rhabdosomes of Climacograptus parvus and the presence 

 of all growth stages of the vesicle in mature rhabdosomes would suggest 

 that these vesicles did not form until a necessity for their formation arose, 

 i. e. when the nemacaulus had been broken and the rhabdosome had become 

 separated from the center of the synrhabdosome. 



There are several other facts leading to the same inference ; as the 

 position of the vesicle at no regular distance from the rhabdosome, but 

 always close to the broken end of the nemacaulus. This is especially 

 noticeable in C. caelatus where sometimes the vesicle forms plump 

 against the growing end of the rhabdosome [see text fig. 22], thereby hin- 

 dering or even preventing further growth. In some specimens the entire 

 nemacaulus becomes covered with small vesicles [text fig. 28], as if there 

 had been a strong endeavor or stimulus to the quick production of numerous 

 supporting vesicles. Numerous such small vesicular appendages have been 

 observed on many individuals in the nemacauli of several other forms, as 

 Glossograptus and Corynoides calicularis [text fig. 29, 30]. The 

 formation of one large vesicle may here have been dissolved into that of 

 many smaller ones. 



4 Finally, the appendages of Climacograptus parvus permit 

 the observation of some structural details of the nemacaulus. 



