668 NEW YORK STATE MUSEUM 



The similarity between the first two species which is indicated by the 

 measurements of the thecae, is still increased by the narrow, rigid and 

 straight character of the branches, and their approximately equal angles of 

 divergence as well in the proximal as also in the distal portions (110°-140° 

 in Bryograptus lapworthi and 110°-130° in D i d y m o gr a p tus 

 nicholsoni). The similarity between the rhabdosoraes in our material is 

 so close that, where only broken branches or young colonies are observed, the 

 distinction between the two is very difficult. 



Didymogr-aptus gracilis is considered by Miss Elles to belong 

 to the same series as D. nicholsoni. There exists indeed an unmis- 

 takable similarity in the habit of the form which is referred in the mono- 

 graph of the British graptolites to D. gracilis Tornquist, to the species 

 of Bryograptus and Didymograptus referred here to the same series. This 

 similar habit is produced by the relative rigidity and straightness of the 

 branches and their similar angles of divergence as well as the slender 

 character of the thecae. We show however in the systematic part of this 

 memoir [p.561 ] that the British form belongs to a different species, which 

 it is here proposed to term D. e lie si. The last represents indeed a 

 derivative of D, nicholsoni with looser arrangement of the thecae; 

 while the typical D. gracilis, which is also present in our fauna, belongs 

 to a different evolutionary series [p.561]. 



(5) D . s p i n o 8 u s is the only species of Didymograptus known to have 

 possessed dorsal and apertural spines. There is however, also a species of 

 Tetragraptus, T. acanthonotus Gurley, found in the Levis shales, 

 which differs from the other species of Tetragraptus by the same features. 

 Yet a comparison of the characters of the thecae brings out the fact that the 

 latter species is a much coarser form, with uniformly wider branches, less 

 closely arranged thecae (which is only coincident with the coarser structure 

 of the whole colony) and more inclined thecae. It is hence doubtful whether 

 the presence of the spines in both species indicates the parallel acquisition of 

 a new character in the related forms or a closer phylogenetic bond. 



