296 PALEONTOLOGY 



arranged in the form of a hollow cone, and are connected 

 at intervals by cross-pieces. This genus ranges from 

 Cambrian to Devonian, but it is only common in one 

 well-defined zone in the Lower Tremadoc. 



The graptolites have proved of the greatest value as 

 zone-fossils. Thanks to them alone, Professor Lapworth 

 was able to unravel the complexities of structure of the 

 Southern Uplands of Scotland, which had been entirely 

 misinterpreted before. Apart from detailed zoning they 

 provide any geologist with the means of recognizing 

 broad divisions : the complex Dichograptidae are easily 

 recognized as marking the lowest Ordovician, the Mono- 

 graptidae as distinctive of the Silurian, and so on. Their 

 zonal distribution appears to be the same in the Ordo- 

 vician in Europe and North America, though in the 

 latter region Silurian graptolites (other than Dendroidea) 

 are very rare : it is only in Australia that an important 

 difference in range has been found (see under Lepto- 

 graptidce, p. 291). 



The graptolites have been assumed to be hydrozoa 

 that is to say, that the individual animal which occupied 

 each theca is supposed to have been a hydvoid polyp, all 

 the polyps in a polypary being united by a living cord in 

 the common canal. A polyp is essentially a cylindrical 

 sac with an opening (the mouth) at one end, and around 

 the mouth a circlet of tentacles; in a hydroid polyp, the 

 internal cavity is undivided. 



There are other fossil hydrozoa besides the graptolites : 

 they form calcareous skeletons and may be spoken of as 

 hydrocorallines (using that term in a less exact zoological 



