5 8 RAMBLES AND REVERIES. 



are developed), and it may be prolonged beyond 

 one or both ends of the colony. The polypary is 

 typically famished at its proximal end with a 

 minnte triangular or dagger-shaped spine (sicula), 

 which represents the embryonic skeleton." 



Graptolites differ from Sertularia in possessing 

 the axis and sicula, while it is quite certain that 

 the typical forms were free swimming and not 

 fixed as Sea-firs always are. 



There are two main groups of Graptolites, one 

 of which (monoprionidian) is characterized by a 

 row .of hydrothecse or cups on one side only of the 

 rod, whilst the other (diprionidian) has a row of 

 cups on each side of the axis. There are others 

 which have four rows of hydrothecae (Tetragraptvs). 

 The diprionidian Graptolites are found in the Cam- 

 brian and Silurian rocks, the ordinary form (mono- 

 prionidian) being found only in the Cambrian. 

 These two forms are illustrated by the ordinary 

 twin Graptolite {Didymograptus Murchisoni} at 

 Fig. 9 and the double Graptolite {Diplograptus 

 pristis) at Fig. 13. 



This discussion respecting the nature and affinities 

 of Graptolites has a very important bearing on 

 the evolution theory. It is significant that while 

 marking so definitely a distinct geological period 

 these interesting fossils appear and disappear with 

 startling abruptness without any apparent cause, 

 revealing such slender relationship to other animal 

 groups as to make it quite impossible to rank them 

 with any other organisms. The believer in the 

 doctrine of special creation may certainly find much 



