566 NEW YORK STATE MUSEUM 



In the same way as Coenograptus is readily traced through Sigma- 

 graptus to Goniograptus, which clearly is a derivative of Clonograptus, the 

 genera Pterograptus and Pleurograptus, at present united with Coenograptus 

 in a small separate group of Dichograptidae, also appear to be traceable to 

 Clonograptus. 



The last remaining genus of the Dichograptidae, represented in the 

 New York fauna, is Temnograptus, a coarse multiramous form which 

 at present can not be connected with any other genus, but, by exclusion, 

 can be said to have been derived from some species of Clonograptus by 

 transitional stages not yet known. 



If we represent the supposed phylogeny of the Dichograptidae in the 

 customary form of a tree and its branches [p.SSS] we find at the base the 

 genus Clonograptus, from which one stem leads up through S t a u r o - 

 graptus dichotomus var. apertus to Bryograptus, another through 

 other forms of Clonograptus. From both of these arise numerous upshoot- 

 ing branches much as in stunted willow trees, some of which at times 

 again send off twigs. If we lay a horizontal plane low down through 

 these bundles of branches, it passes through forms which, though belonging 

 to various offshoots or lines of descent, all have in one bundle the character 

 of a Bryograptus and in the other that of a Dichograptus or a Groniograptus. 

 Laying a higher horizontal plane through the branches, we intersect the 

 forms all in the Tetragraptus stage, and the ultimate twigs are nearly all in 

 the Didymograptus stage. 



These horizontal planes represent a certain stage of development as for 

 instance the Tetragraptus stage, passed through by all the series repre- 

 sented here as upshoots of Dichograptidae at approximately the same time 

 in geologic history. At the intersections of the plane with the branches 

 we find hence the species which, while belonging to different races, have 

 all reached the same stage in the gradual reduction of the number of 

 branches. These species have been grouped into the genera as they are 

 at present understood. We have here, therefore, a remarkably distinct case of 

 the law of parallelism in development. 



