564 NEW YORK STATE MUSEUM 



and subradial arrangement o£ the thecae, but also a differentiation of tlie 

 character of the apertures, which manifests itself principally in the vary- 

 ing development of the apertural mucros or spines. 



In the absence of other species of Tetragraptus, which could be con- 

 sidered radicles of species of Phyllograptus, and the distinct succession of 

 the forms in time, it is more probable that the Phyllograptidae form a 

 compact, monophyletic group. Yet even in this small group it is evident 

 that Phyllograptus ilicifolius and its successor, P , anna, are 

 more closelj^ related with each other than Avith P. angustifolius and 

 P. typu s. 



A peculiar branch of the Dichograptidae, on whose genetic relations 

 hitherto no light has been thrown, are the Coenograptidae. These do not 

 appear till the Upper Champlainic, or Ordovicic horizons are reached. The 

 path of their derivation has been pointed out by the discovery of an ancestral 

 type of the family, viz Sigmagraptus praecursor [pl.5, fig.l3] in 



the Deep kill fauna. This interesting form pos- 

 sesses two principal stems, from which undivided 

 branches originate alternately on either side. The 

 principal stems foreshadow already in their curva- 

 ture the peculiar sigmoidal curve of those of Coeno- 

 graptus. In Coenograptus the branches of one half 

 Fig-. 15 coenograptus gra- of ^ rhabdosomc are turned to one side, those of 



cilis Hall sp. x4 (Copy from ' 



Ruedemann) ^^^ other, tumed to the opposite side. On well 



preserved specimens of C. gracilis [see text figure], I have however 

 been able to observe that every second branch bends across the principal 

 stem to the same side as the preceding branch, which makes the original 

 arrangement of the branches identical with that of Sigmagraptus. Both 

 have also in common the long, little overlapping, slender thecae and thin 

 branches. 



A peculiar feature of Coenograptus as well as of Sigmagraptus is the 

 divergence of the principal stems near the middle of the sicula, which 

 leaves the apertural end of the sicula protruding fully beyond the stems. 



