a Family of Graptolites. 



23 



separated. The eight- branched forms, with some abnormally deve- 

 loped examples, are referred to B. octobrachiatus, Hall. The forms 

 furnished in the typical state with twelve branches are to be regarded 

 as a distinct species, and that these examples cannot be referred 

 either to D. octobrachiatus (Fig. 7) or to D. Logani, Hall, seems to the 

 author to follow from the facts that in some localities only eight- and 

 many-branched forms (D. Logani, Hall) have been found {e.g. 

 Australia and Canada), in others only those with numerous branches 

 (e.g. the Australian localities examined by E. Etheridge, jun.), and 

 in other localities again only eight-branched forms. In the locality 

 investigated by the author (Galgenberg, Christiania) the forms with 

 8 and 12 branches occur together, and he considers that here two 

 different species are associated. Further, the twelve-branched forms 

 constitute a bilaterally symmetrical species, and the number of 

 normally developed examples as compared with abnormal ones seems 

 to indicate their distinctness. 



Fig. 8. Dichograplus Kjerulfi, Herrmann, Phyllograptus-skales, Galgenberg, 

 Christiania (natural size). 



Description. — Hydrosoma bilaterally symmetrical, consisting of 12 

 branches when typically developed. From a sicula, which usually 

 persists as a rounded knob, issue 2 short branches, which bifurcate 

 first into 4 and then into 8 branches. Of these 8 branches, 4 again 

 divide, and these four are almost always those situated furthest from 

 the plane of symmetry, i.e. the inner branches of each half of the 

 hydrosoma of an octobrachiate form (Fig. li). The branches are 

 slender, thinnest in the neighbourhood of the sicula. The full 

 number of branches is frequently not attained, and rarely exceeded. 

 The central part of the hydrosoma is almost always enclosed in a 

 chitinous disc. The hydrothecse are rarely to be seen within the 

 disc. There are 9 of them in a length of 10 millim. At the proxi- 

 mal end they form with the axis an angle of 13° — 15°, at the distal 

 end one of about 35°. They are three times as long as broad, and 

 about one-third free. The lower border is curved. Aperture-angle 

 55°. 



