554 NEW YORK STATE MUSEUM 



of the Dichograptidae, as Tetragraptus, Didymograptus, Loganograptus, 

 Dicliograptus, are largely defined, is a character of minor importance. 



They find that, when the Dichograptidae are separated into groups 

 characterized by their thecae, (1) the different groups exhibit a series of 

 parallel modifications as regards the number of branches in the rhabdosome, 

 and (2) the older forms of the group are more complex, and the later forms 

 undergo reduction in the number of branches. 



To illustrate this important principle, the authors show that, of the nine 

 species of Tetragraptus well known to them, " eight are represented by forms 

 of Didymograptus which are closely comparable as regards the characters of 

 the hydrothecae." It is further stated, that four of these four branched 

 Tetragrapti are represented, as regards the character of the thecae and the 

 amount of the angle of divergence, by forms of Dichograptus or Bryograptus. 



The extraordinary resemblances between the various species of Bryo- 

 graptus, Dichograptus, Tetragraptus and Didymograptus have led the authors 

 to the conclusion that the species of these genera have not descended from a 

 common ancestral form for each genus, but are the result of the variation of 

 a number of different ancestral types along similar lines. These hetero- 

 genetic, homoeomorphous derivations, as they are termed by the authors, 

 appear more or less simultaneously, a fact, which has made them extremely 

 valuable to the geologist. The explanation for the remarkable parallelism 

 displayed in the reduction of multiramous, irregularly branching forms 

 through multiramous regularly branching to pauciramous symmetric forms, 

 among the Dichograptidae, is sought in the suggestion that symmetry 

 in the arrangement of the branches would tend to insure an equal supply 

 of food to each branch, and that the fewer the branches the greater the 

 supply of food to the entire organism. 



The suggestions of Nicholson and Marr on the phylogeny of the 

 Dichograptidae have been made the subject of a most detailed investigation 

 by EUes [1898, p.529ff]. This authoress, who states that in her study of the 

 Skiddaw slate graptolites she has also been greatly impressed by the 

 remarkable resemblances between species of different genera, has obtained 



