562 NEW YORK STATE MUSEUM 



In Didymograptus filiformis and gracilis this feature is some- 

 what modified in so far as the two branches originate in unequal distances 

 from the apex of the sicula, one in the middle and another a little more 

 distally. 



Didymograptus gracilis presents thin branches, long, narrow, 

 little overlapping, remote thecae, similar to the members of this group ; and, 

 also, its first two thecae originate at different levels as in D. filiformis. 

 It is, nevertheless, doubtful whether it represents more than a later paral- 

 lelism to the forms of this series. 



A comparison of the number of dichograptid species cited in the fore- 

 going discussion and the considerably greater number of forms known 

 from the Lower Champlainic graptolite beds, proves readily that still a 

 considerable number of species, specially of the genera Tetragraptus and 

 Didymograptus, have not been connected with older forms with a greater 

 number of branches. Such a comparison will also show that the number 

 of species increases considerably as we proceed from the earlier multi- 

 ramous Clonograptus and Bryograptus toward Didymograptus. This 

 stands, of course, in accordance with the general fact of the multiplication 

 of types during the time of the progressive development of a race. As, 

 hence, the number of known species of Didymograptus is considerably 

 greater than that of Tetragraptus, it is evident that not every Didymo- 

 graptus can have its Tetragraptus, and that either several species of Didymo- 

 graptus have developed from the same species of Tetragraptus or 

 species of Didymograptus have become themselves, the radicles for new 

 species of the same genus. The latter alternative is strongly suggested 

 by the great similarity of several species of Didymograptus among 

 themselves, as among D. affinis and D. nicholsoni; or 

 between D. extensus and D. nitidus; or between D. (gib- 

 berulus) caduceus and D. forcipiformis. 



Our observation of the similarity between Clonograptus 

 (Staurograptus) dichotomus and certain species of Bryograptus 

 and of the fact that a laterally compressed Clonograptus can only with 



