184 



DISTRIBUTION. 



discovery of BliaMoplcurn renders us acquainted witli a polyzoon in whose test is developed a 

 cbitinous rod in almost all respects like that of the graptolites.^ 



The graptolites are exclusively of Silurian age. They occur first in Lower Silurian beds, 

 and unless we include among them the anomalous genus I)cndro(/raptus, which has been obtained 

 from the Potsdam formation of North America — the probable equivalent of our Lingida Flags — 

 their lower limit will be the summit of the Lingula Flags, from which they range into the Ludlow 

 of the Upper Silurian.^ 



The diprionidian, or double-rowed forms occur in great abundance in the Lower Silurian 

 Rocks, where they are associated with monoprionidian or single-rowed forms. Graptolites 

 become much more rare in the Upper Silurian, where they are represented exclusively by some 

 monoprionidian species.^ 



The following table will show the geological formations which have afforded evidence of the 

 former existence of the Hydroida. 



^ The comparison of the rod of Rhabdopleura to tliat of a graptolite has already been made by 

 Dr. Nicholson (' Manual of Zoology'), though he adopts the more generally accepted view which sees 

 in the denticles of a graptolite the hydrothecc'e of a hydroid. 



^ The curious Fenestella-Wke genus Bictyonema has been referred to the Grapiolitida by Hall. 

 It ranges from the Lower Silurian into the Middle Devonian ; but whatever may be the nature of this 

 remarliable fossil, it is almost certainly not a graptolite. Schiraper (' Traite de Paleontologie vegetale ') 

 places Dicii/onema along with Oldhamia among the nullipores, 



^ If Reiiolites, Bar., be a true graptolite we must then carry the Diprionidian forms into the 

 Upper Silurian, for this genus occurs in the Llandovery and Upper Wenlock formations. RetioH/es, 

 however, is an exceedingly anomalous form, and departs in so many respects from the typical Grapto- 

 lites as to render the propriety of associating it with those fossils extremely doubtful in the present 

 state of our knowledge. 



* See preceding note. 



