HYDROZOA. GRAPTOLITOIDEA 



67 



larger the number of branches the smaller would be the 

 total supply of food for each individual of a colony ; conse- 

 quently, when one species of a four-branched type {Tetra- 

 graptus) had become a two-branched form, it would have 

 a considerable advantage over the remaining four-branched 

 forms, which would, as a result, soon die out. 



Didymograptus. (fig. 17.) Polypary bilaterally sym- 

 metrical, consisting of two uniserial stipes diverging at an angle 

 which varies, in different species, from 0° to 180° (or occasionally 

 more). Hydrothecse subcylindrical, in contact for a considerable 

 part of their length. Lower Arenig to Upper Llandeilo. Ex. D. 

 murchisoni, Lower Llandeilo ; D. patuhis, Arenig. 



Phyllograptus. (fig. 20.) Polypary leaf-like, consisting of 

 four uniserial stipes united along the 

 whole of their length. Hydrothecae 

 cylindrical or subcylindrical, in con- 

 tact throughout their entire length. 

 Sicula pointing distally. Arenig. 

 Ex. P. typus. 



Tetragraptus. (fig. 14.) Poly- 

 pary bilaterally symmetrical, uni- 

 serial, consisting of four simple radi- 

 ating branches which arise from the 

 bifurcation of two short branches 

 coming off from opposite sides of the 

 sicula (constituting a Didymograptus stage). Hydrothecae cylindrical 

 or subcylindrical, in contact for a considerable part of their length. 

 A central disc may or may not be present. Arenig. Ex. T. quadri- 

 brachiatus. 



Dichograptus. Polypary typically bilaterally symmetrical 

 consisting of eight uniserial main stipes produced by bifurcation 

 through Didymograptus and Tetragraptus stages. Hydrothecae 

 cylindrical or subcylindrical. A central disc is frequently present. 

 Lower Arenig. Ex. D. octobrachiatus. 



Loganograptus (Arenig) and Clonograptus (Tremadoc and Arenig) 

 are forms in which bifurcation has proceeded further than in Dicho- 

 graptus. 



5—2 



Fig. 20. Phyllograptus, Arenig 

 Rocks. The polypary has 

 been cut in two, and the upper 

 part raised so as to show the 

 four branches. Natural size. 



