CCELENTERATA — HYDROZOA 1 1 7 



is suggested by the reversion of direction of the cups in a col- 

 ony, the embryonic cup or sicula growing in a direction oppo- 

 site to the rest of the cups (20). 



Graptolites were abundant during the upper Cambrian, the 

 Ordovician and the Silurian, while a few stragglers continued 

 to exist into the Mississippian. 



Derivation. — Graptolithida > Greek graptos, written, + 

 lithos, stone. The resemblance of these fossils, especially those 

 of the sub-order Graptoloidea, to the ancient writings on stone 

 suggested not only the name of the order but also the latter 

 part (graptus) of most of the generic names of graptolites. 



The graptolites (Order Graptolithida) are sub-divided as follows: 



Sub-order a, Dendroidea. — Colony (rhabdosome) branching 

 irregularly in a funnel or fan-like manner. Cups (hydrothecae) 

 usually pits upon the branches. 



Probably most of the Dendroidea, especially the heavier and 

 more shrub-like forms, were sessile in an upright position, 

 attached to some object at the sea-bottom ; this would likewise 

 account for their local distribution. Some have adhesive 

 threads (hydrorhizae) , like Sertularia and other modern sessile 

 hydroids. Many of the species of Dictyonema, on the other 

 hand, have a very wide distribution, and, at least in their 

 younger stages, floated about attached to a supporting disk. 



The name is from the tree-like (Greek dendron, a tree) method 

 of growth of the colony. 



Dictyonema (Fig. 41). Cambrian-Mis sis sip plan. 



Colony (rhabdosome), a rapidly expanding fan or cone with 

 cups upon the inside. The slender branches are united at short 

 intervals, producing a net-like appearance (w^hence the name 

 from the Greek dictyon, net, + nema, thread). When young, 

 the rhabdosome was suspended from a long, thin thread attached 

 to a chitinous disk; later (at least at times), it was sessile by 

 rootlets. D. flabelliforme is common in the earliest Ordovician 

 rocks of eastern North America and western Europe, 



