22 NORTH AMERICAN INDEX FOSSILS. 



ture is produced [Dictyonema). When the original buds from the 

 sicula are completely turned back upon themselves, so as to open 

 in the opposite direction from the sicula, and grow together, 

 back to back, a double-rowed structure is produced with hydro- 

 thecae on opposite sides of the median line. Types of this kind 

 (Diplograptus y Climacograptus) are reenforced by a median axis or 

 rod, the virgula, which extends the entire length of the branch and 

 beyond the last-formed hydrothecae. It generally appears as a 

 shining, solid rod, extending beyond the hydrothecae. A number 

 of types with a single row of hydrothecae also possess a virgula. 

 Types possessing this structure are classed in a division by them- 

 selves (Axonopkora). In all cases the colony derived by budding 

 from a single sicula is called a rhabdosome, whether consisting of 

 one (single- or double-rowed) or many (single-rowed) branches. 

 In many cases a number of rhabdosomes may be united by the 

 ends of their virgulae (that farthest away from the sicula), forming 

 a compound rhabdosome, which in some cases is provided with a 

 float or air-bladder-like structure, from the base of which also de- 

 pend the gonangia or reproductive sacs (Diplograptus). (Fig. 54.) 

 While some of the Graptolites were probably attached to sea- 

 weeds like modern hydroids, others were free-floating or plank- 

 tonic organisms. This and the fact that sea weeds when torn from 

 their anchorage would also float, explains the wide distribution of 

 the Graptolites and their presence in rocks of the same age in 

 widely separated parts of the world. They are best preserved in 

 fine mud-rocks, especially the black shales. They have, however, 

 been found in limestones (calcilutytes) and even in sandstones, 

 They are excellent index fossils of the lower Palaeozoic rocks. 



Literature. 



Some of the more important works on American Graptolites 

 are: 

 1865. Hall, James. Graptolites of the Quebec group. Canadian 



Organic Remains Decade II., Geol. Surv. Canada. 

 1868. Hall, James. Introduction to the Study of the Graptolites. 



N. Y. State Cab. Nat. History, 20th Ann. Report. 

 1879. Spencer, J. W. Graptolites of the Niagara Formation. Can. 



Naturalist, 1878-79, p. 457. 

 1884. Spencer, J. W. Graptolites of the Upper Silurian System. 



Mus. Univ. Missouri Bulletin. 



