Development and Mode of Growth of Diplograptus. 227 



by assuming that the sediment which gathered around the cen- 

 tral organs and under the ascending rhabdosomes, caused the lat- 

 ter to be buried finally at a somewhat higher level than the 

 central disc. This explanation, however, presupposes that the 

 central parts were attached to the ground. There have also been 

 observed quite a number of siculas, the basal appendages of which 

 lie in another level of the matrix and appear, therefore, on the 

 surface of the slabs in a pit or on a little node. 



The argument has been repeatedly advan ced that because of their 

 rigidity the Graptolites can hardly have been adapted to a pelagic 

 mode of life. The profuse occurrence of broken rhabdosomes of 

 Diplograptus throughout the Utica slate is sufficient proof that 

 the hydrocauli and rhabdosomes of Diplograptus possessed only 

 a very slight flexibility. Such a lack of flexibility must have 

 endangered the colonies wherever the water was moved. But 

 there must have been motion in the depths in which the sedi- 

 ment constituting the Utica slate settled, for the broken rhabdo- 

 somes on most slabs lie in a parallel direction ; hence the relative 

 scarcity of entire colonial stocks in comparison with the enormous 

 multitudes of broken rhabdosomes. The two localities near 

 Dolgeville which furnished the colonies of the two species of 

 Diplograptus would then represent areas which were free of dis- 

 turbing bottom currents at the time of the formation of the thin, 

 colony-bearing intercalations. 



If the colonies of Diplograptus were indeed moored in the 

 mud, the organ which I compared with Hall's central disc, would 

 have been much too small to serve as an apparatus for fixation. 

 The question of the means of fixation and the function of the 

 vesicle described in this chapter and termed the " basal cyst,'^ 

 would arise. May the latter, perhaps, not have been the top- 

 most part of the colony, as supposed by the writer, and the ver- 

 tical order of organs, from below upwards, have been ; basal cyst, 

 gonangia, rhabdosomes? May it, further, have been a con- 

 trivance, which, by being buried in the detritus, served to pro- 

 cure that stability for the colony which otherwise only a large 

 disc like the central disc of the Monoprionidce could have pro- 

 vided on the soft, loose ooze ? Since the discovery above men- 

 tioned, the writer has not had opportunity to study the material 

 of Diplograptus so* thoroughly with reference to this problem 

 as will be necessary. 



