JHE HYDROMEDUSAE 



51 



between its members, the canals communicatinj^ below with each other 

 and witli the sicula. Tlie sicula faces in a direction opposite to tliat of 

 the other thecae. 



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

 both Monoprionidae and Diprioiiidae, 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 apjiear from their distribution that 



Ki(i. .")(>. 



Diagrams illiistratins the strncturn and developiiiPiit of Graptolites. 1, Jt^nor/raptus 

 priniinii (after Nicliol.s(jn) ; 2, l()ii},'it\iiliMal, and 3, transverse sections of Monngraptus 

 jtriodon : 4, transverse section of I'liyllivjrdiitiis ; 5, base of colony of Didjimoqrnptits minutvs, 

 a two - brancheil Monoprionidan (after Winian); (i, dntptolites with supposed (;onangiuin 

 (Dawsonia) in l)lace (after Iloernes) ; 7, base of colony of Diplngniptus (after Wiinan) ; 8, part 

 of colony of Itiliolitcs, the perisarcal niesliwork has been left on one tlieca only (after Holm): 

 9, transverse section of lletiolltex, showini; two thecae, the common canal, and the perisarcal 

 nieshwork growing oiit from the lip of each theca. CC, common canal ; S, sicula ; ^S", mouth of 

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

 cessively fonned thecae, H being the iirst fornieil. 



the Diprionidan forms are the older, the Monoprionidan forms having 

 arisen by the suppression of a row of thecae. 



Order 3. Retinlniiha (Fig. 56 '*'^). 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. 



