THE HYDROMEDUSAE 



between its members, the canals communicating below with each other 

 and with the sicula. The sicula faces in a direction opposite to that of 

 the other thecae. 



There is some ground, at present most insecure, for the belief that, in 

 both Monoprionidae and Diprionidae, the individual stems were united 

 into colonies, and sprang from a central mass, the sicula being at the distal 

 end of each stem. The Graptoloids range from the Lower Arenig beds up 

 to the Silurian inclusive, and it would appear from their distribution that 



rt LVYU- UIcUlUUCU lTlUUl_H.HlUllmalI ^dllAvl M llllttllj , \Jj IT ( tt^frut tfrCO VY 1 Kll DUTn/DVU ,VJIlaugl ulll 



( Dawsonia) in place (after Hoernes) ; 7, base of colony of Diplograptus (after Wiman) ; 8, part 



cessively formed thecae, S being the first formed, 



the Diprionidan forms are the older, the Monoprionidan forms having 

 arisen by the suppression of a row of thecae. 



ORDER 3. Retioloidea (Fig. 56 8 - 9 ). This group, which is well 

 developed in the Ordovician rocks, includes Graptolites which have no 

 true sicula, but are characterised by the periderm forming an open mesh- 

 work. The thecae are generally arranged in two series ; one or two 

 'virgulae may be developed. 



