HYDROZOA. GRAPTOLITOIDEA 



59 



genera, and sometimes even in different species of the 

 same genus; but in any one species they are usually 

 similar, except that they diminish in size towards the 

 proximal end of the polypary; they may resemble the 

 sicula in shape (Didymograptus), or they may be tubular 

 {Rastrites), prismatic (Diplograptus), conical (Monograptus 

 tricing ulatus), or coiled {Monograptus lobiferus). They 

 may be in contact throughout their entire length (Phyllo- 

 gvaptus), at their bases only (Nemagraptus), or, in a few 

 cases, entirely separate (Rastrites). Frequently they are 

 provided with one or more spines _.-w 



near the mouth. In most grapto- 

 lites the hydrothecse communicate 

 freely with the common canal, and 

 in this respect differ from living 

 hydroids, in which there is a 

 constriction or an imperfect dia- 

 phragm at the base of each 

 hydrotheca, separating it from the 

 common canal (fig. 12); but some 

 specimens of Didymograptus and 

 Tetragraptus seem to show evi- 

 dence of a septum between each 

 hydrotheca and the common canal. 

 A microscopic examination of 

 thin sections of Monograptu s shows 

 that the periderm consists of three 

 or four layers, the external and 

 internal layers being much thinner 

 than the others. In Retiolites the 

 middle layer of the periderm is 

 formed of a network of fibres (fig. 15) whilst the inner and 

 outer layers are very thin. 



B 



w-' x 



Fig. 15. Retiolites geinitzi, 

 Silurian. A, section across 

 polypary. B, proximal end 

 of polypary with the outer 

 layer removed. Enlarged 

 (after Holm). ?c, .r, rods in 

 the network formerly re- 

 garded as virgulae. 



