GRAPTOLITES OF NEW YORK. PART 1 569 



mucii surpassing that of the dependent, declined or reclined forms. This 

 is exemplified specially by Didymograptus extensus, nitidus 

 and p a t u 1 11 s . As my material shows that these forms also pos- 

 sessed but an extremely short, if any, nema, and that the primary disk, 

 which was large, was closely aiRsed to the sicula and to the center 

 of the colony, the inference is very plausible that these long branches 

 were closely adhering to the underside of floating objects, perhaps seaweeds, 

 similarly as the colonies of some bryozoans are at pi*esent. In the Upper 

 Champlainic appear also dependent, or rather flexuous types of Didymo- 

 graptus with branches of astonishingly large linear dimensions. 



Another tendency of development, becoming manifest among the 

 Dichograptidae and incidentally mentioned above, is that of condensing the 

 branching by a close arrangement of the thecae. This is largely accom- 

 plished by giving the thecae a more oblique position to the axis of the 

 branch. The gradual inci'ease in the angle of inclination in several series has 

 been pointed out above [p.559]. 



The branches of several species recapitulate this process in their 

 individual ontogenetic development, as we have shown in a former paper 

 [1902, p.587] in the cases of Goniograptus thureaui and Tetra- 

 g r a p t u s f r u t i c () s u s . In these forms the branches begin Avith long- 

 slender thecae, with a small angle of inclination. As a rule gradually, but 

 sometimes (juite abi'uptly, as in T . f r u t i c o s u s , the thecae become 

 more closely arranged by a decrease of overlap, and moi'e inclined. In the 

 later species they also become provided with apertural mucros, Avhich must 

 be a later acquisition of the series [see fig.ll, p.531]. 



The cause of the condensation of the branches is to be found in the 

 reclined position assumed by them in an endeavor of the rhabdosomes, 

 which have become suspended, to restore to the zooids their original erect 

 position. By this ascending growth of the branches, they lengthen in the 

 direction of the point of fixation or support, whatever this may have 

 been, thus necessitating a corresponding lengthening of the supporting 

 nema and thereby endangering, b}- imdue longitudinal growth, the fixation 



