The tuning fork graptoloid, Didymograptus murchisoni (Beck, 1839)

The development of D. (Didymograptus) murchisoni (Beck), the type species o f the genus, is shown to be unequivocally of artus type, with th1(1) dicaly al and a low metasicular origin of th1(1). An evolutionary line from the ea rly, Arenig, bifidus group is supported; and clear distinctions are drawn b etween the subgenera D. (Didymograptus), D. (Didymograptellus) and D. (Jenk insograptus), the first of these being redefined. The range of variation of biocharacters of D. murchisoni is analysed using exquisitely preserved und eformed material from Saudi Arabia, and comparisons are made with the undef ormed type material, and with undeformed (nodule) collections from Abereidd y Bay, Pembrokeshire. The astogenetically late developments of D. murchison i are described.