568 NEW YORK STATE MUSEUM 



As, now, according to Lapworth's theory, the Dichograptidae were 

 suspended forms, derived from sessile benthonic forms, it seems necessary 

 for us to take into account the influence of the suspension from a mostly 

 very thin thread — often indeed, as in Tetragraptus bigsbyi, etc., 

 strikingly thin in relation to the size of the colony — on the gradual production 

 of the symmetric arrangement of the branches. An irregular growth in a 

 suspended colony is clearly liable to lead to a disturbance of the equilibrium 

 of the colony, the consequent sinking of one half and rising of the opposite 

 half of the rhabdosome and a resulting disarrangement of the normal position 

 of the thecae. Tlie nicety of balancing is hence quite plausibly one of 

 the ends sought in the often rigidly symmetric arrangement of the 

 branches. 



The reduction of the number of branches, however, goes hand in hand 

 with a lengthening of the remaining branches, so that the total length of 

 the branches or of the number of thecae in the later pauciraraous forms 

 is not only not smaller but materially greater than in the early multi- 

 ramous forms; for we see everywhere the numerous short branches of 

 the Clonograptus and Bryograptus forms followed by the immensely long 

 branches of the species of Loganograptus, Dichograptus, Temnograptus, 

 Coenograptus and Tetragraptus. We can not for this reason believe that 

 the reduction of the number of branches could have been for the purpose 

 of increasing the food supply of the entire organism, but incline rather to 

 the belief that this reduction was incidental to the assumption of the symmetric 

 form. 



If we contrast Clonograptus with its great power of branching by 

 dichotomy with the last sprouts of the i-ace, the species of Azygograptus, 

 which are altogether unable to produce dichotomies, it appears that the 

 tendency to a reduction of the number of branches finally became so fixed 

 that the series shot, so to say, beyond the mark, and the power of branching 

 was finally lost altogether. 



A peculiar feature of the species of Didymograptus is, that those which 

 are rigidly horizontal attain, at least in the Lower Champlaiaic fauna, a size 



