HYDROZOA. GRAPTOLITOIDEA 69 



Climacograptus. Polypary biserial. Hydrothecse tubular, 

 with sigmoid curvature, apertures placed in depressions. Sicula 

 often concealed. Upper Arenig to Tarannon. Ex. C. normalis, 

 Llandovery and Tarannon. 



Dimorphograptus (see p. 58). Llandovery. 



Monograptus. (fig. 13 c.) Polypary unbranched, uniserial ; 

 straight, curved, or spiral. Hydrothecse vary in form in different 

 species. Sicula attached to the proximal end of the polypary, and its 

 j)ointed end directed distally. Lower Llandovery to Lower Ludlow. 

 Ex. M. nilssoni, Lower Ludlow ; M. leptotheca, Llandovery ; M. 

 priodon, Wenlock ; M. spinigerus, Llandovery. 



Rastrites. Similar to Monograptus, but the hydrothecse are 

 long, tubular, and widely separated. Llandovery to Tarannon. Ex. 

 R. peregrinus, Llandovery to Tarannon. 



CyrtOgraptUS. Similar to Monograptus, but coiled into a 

 plane spiral with branches given off from the external (hydrothecal) 

 margin. Upper Tarannon to Lower Ludlow. Ex. G. murchisoni, 

 Wenlock Shale. 



Retiolites. (fig. 15.) Polypary biserial, straight. Hydrothecae 

 like those of Diplograptus. Periderm consists mainly of a network 

 of threads and rods. Lower Bala to Wenlock. Ex. R. geinitzianus, 

 Upper Tarannon and Wenlock. 



Distribution of the Graptolitoidea 



In Britain the earliest graptolites occur in the Tremadoc 

 Beds, where we find the genera Bryograptus and Clono- 

 graptus ; in the Olenus-shales of West Gothland Dicho- 

 graptus also occurs. The last two forms are also present 

 in the lower part of the Arenig Beds, but other genera 

 such as Tetragraptus and Diclymograptus are associated 

 with them and soon become abundant ; Phyllograptus also 

 occurs here. In the Llandeilo, the graptolites are transi- 

 tional between the Arenig and Bala forms, Phyllograptus 

 is absent, Didymograptus is fairly common in the lower 



