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 steins 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 



FIG. 50. 



Diagrams illustrating the structure and development of Graptolites. 1, Monngraptus 

 priodon (after Nicholson) ; 2, longitudinal, and 3, transverse sections of Monograptus 

 priodon; 4, transverse section of I'hyllograptus ; 5, base of colony of Didifmograptns minutus, 

 a two - branched Monoprionidan (after Winian) ; ti, Graptolites with supposed gonan^ium 

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

 of colony of Rrtiolites, the perisarcal meshwork has been left on one theca only (after Holm); 

 9, transverse section of RetioUtc*, showing two thecae, the common canal, and the perisarcal 

 meshwork growing out from the lip of each theca. C'C, common canal ; S, sicula ; S', mouth of 

 sicula; T, theca; V, virgula ; V, zigzag virgula of Jietiolites; II, III, IV, etc., indicate suc- 

 cessively formed thecae, S being the tirst 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 V). 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. 



