48 HYDROZOA. GRAPTOLITOIDEA. 



Climacograptus. Polypary diprionidian. Hydro- 

 thecse placed vertically and separated by deep depressions. 

 Virgula prolonged beyond both extremities. Lower 

 Arenig to Tarannon. 



Phyllograptus. Polypary leaf-like, formed of four 

 monoprionidian branches united along the whole of their 

 dorsal surfaces. Hydrothecae rectangular, in contact 

 throughout their entire length, and furnished with two 

 strong spines. Arenig. 



Retiolites. Polypary simple, diprionidian, narrower 

 at the ends. There are two virgulas, which are feebly 

 developed, one being placed in each of the lateral faces of 

 the polypary. Hydrothecse rectangular, alternating, given 

 off from a single common canal. Inner layer of the 

 periderm is in the form of a chitinous network. Lower 

 Bala to Wenlock. 



Distribution of the Graptolitoidea. 



The oldest graptolites in Britain occur in the Tremadoc 

 Beds, where we get the genus Bryograptus ; in the 

 OZewws-shales of Gothland, Dichograptus and Clonograptus 

 also occur. These forms are also found in the lower part 

 of the Arenig Beds, but other genera are associated with 

 them and soon become abundant, such as Tetragraptus 

 and Didymograptus ; Phyllograptus also occurs here. 

 The Llandeilo graptolites are transitional between the 

 Arenig and Bala forms, Phyllograptus has died out, 

 Didymograptus is fairly common in the lower part, and 

 the genera Dicellograptus, Dicranograptus, Diplograptus 

 and Climacograptus now appear for the first time. In 

 the Bala Beds, these four last-mentioned genera become 



