GRAPTOLITES OF NEW YORK, PAUT 1 667 



Some peculiar side branches are, that leading through Goniograptus 

 and Sigmagraptus to Coenograptus, another producing through Tetra- 

 graptus (bigsbyi)8imili8and others the Phyllograptidae, and the 

 branch represented by the Temnograptidae. 



The appended diagram is intended to illustrate these supposed phylo- 

 genetic relations of the species of the Dichograptidae and the character of 

 the present genera as stages in a parallel development of \arious series. It 

 is based on Nicholson's and Marr's, and EUes's suggestions, the phylogenetic 

 diagram furnished by the last named author, and the observations of phyletic 

 series in the New York fauna, cited above. We have, for reasons stated 

 before, referred all forms to Clonograptus as a radicle, added the series 

 recognizable in our fauna and restricted ourselves as much as possible to 

 forms found either here or in the Canadian extension of the beds. 



d Supposed causes of the evolution of the Dichograptidae. A study of 

 the stages through which the various series of Dichograptidae pass during 

 their parallel development shows that the whole race begins with multi- 

 ramous, very irregularly branching forms and ends with pauciramous, 

 very regularly branching and symmetric forms. This tendency makes itself 

 manifest whether we follow a series leading from the Cambric Clonograptus 

 through Goniograj)tus to Sigmagraptus and Coenograptus, or from the 

 same genus to Temnograptus, or through Bryograptus to Tetragraptus and 

 Didymograptus, or, finally, through Dichograptus to the same last named 

 stages of development. 



As explanation for this tendency, Nicholson and Marr siiggest its 

 connection with the supply of food. They argue 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. 



It strikes us that this suggestion does not take sufficient notice of the 

 irregular branching, multiramous habit, insisted on by the Dendroidea, 

 which, as Dendrograptus, were attached by thick stems, while such Dendroidea 

 as Dictyonema flabelliforme, which were clearly suspended, 

 become symmetric in their growth. 



