342 



The true graptolite colony ( Rhabdophora ) begins with a 

 small conical cup or hydrotheca, which is known as the 

 sicula. 



The base of this cup is prolonged into a slender thread, 

 the nema,, which in many forms serves as an organ for 

 attachment. In the earlier genera, the second and succeed- 

 ing hydrothecae continue to grow in the same direction as 

 the sicula, so that if the colony was attached to floating 

 objects by the nema, the openings of the hydrothecae all 

 pointed downwards. In later genera the second and suc- 

 ceeding hydrotheca? open in the opposite direction from the 

 sicula, having attained this condition through intermedi- 

 ate stages, and as a result of the permanent attachment of 

 the colony to floating objects. The nema in these grapto- 

 lites becomes prolonged, and constitutes the virgula or 

 axial supporting rod, the tip end of which is fastened to the 

 supporting object. Thus, the new hydrotheca? come in, suc- 

 cessive! v. Viet ween the sicula and the attached end of the 

 prolonged nema, ranging themselves along this prolonged 

 nema for support. 



Some graptolites appear to have led a holo-planktonic 

 existence, the nema being attached to a central organ or 

 disk, which probably served as a float. This was long ago 

 demonstrated in a number of species by Professor Hall, and 

 lately has been shown in great detail in Diplograptus by 

 Ruedemann. Whether holo-planktonic or pseudo-planktonic, 

 either method of life accounts for the wide distribution of 

 the graptolites. The fact that they are almost universally 

 found in carbonaceous shales suggests that floating alga' 

 may have been the principal carriers of these organisms, the 

 decaying vegetable furnishing the carbon for coloring the 

 muds in which the organisms were buried. On the other 

 hand, it is not improbable that much of the carbonaceous 

 material was derived from the graptolites themselves. The 

 general slight thickness of these beds, and the fact that in 

 successive \^W the species change, indicate a slow accumu- 

 lation of the deposits in relatively quiet water. 



