REPORT OF THE STATE PALEONTOLOGIST 1901 589 



the thecae within the branches by figuring a remarkably large 

 and well developed specimen of Tetragraptus frutico- 

 sus (fig. 15) and a smaller new species of Tetragraptus, which 

 eventually will be described asT. taraxacum (fig. 16). The 

 latter species is characterized by remarkably slender proximal 

 thecae and an abrupt change to erect, broad distal thecae. 



Fig. 16 Tetragraptus tar- 

 axjacum sp. nov. Snows 



abrupt change from narrow Pig. 17 Didymograptus (Leptogr apt us) sp. nov. Pos- 



to wide thecae. x3J^ sesses short proximal and long distal thecae. x3^| 



That the direct opposite pattern to this compact structure 

 represented by Tetragraptus and Isograptus, namely the ex- 

 tremely slender and graceful colonies of Leptograptus with their 

 long filiform thecae, are likewise derived from a form having the 

 Bryograptus type of thecal structure, is shown by the colony 

 of a new species of Leptograptus from the Deep kill beds, which 

 is represented in fig. 17. Here we have in contrast to the onto- 

 genetic changes noted before, tubular proximal thecae, suc- 

 ceeded, as the branches lengthen, by extremely thin and long 

 distal thecae which hardly deviate from the direction of the axis 

 of the branch. The outgrowth of such extremely different, mor- 

 phologically contrasted branches as those of Tetragraptus 

 fruticosus, or T. taraxacum and of this Lepto- 

 graptus from the identical type of proximal brachial thecae 

 is certainly a strong argument for the propriety of viewing the 

 cmanges within the branches as being of ontogenetic nature, 

 and of corresponding phylogenetic importance. 



Furthermore, the fact that the thecae within the same colony 

 show a gradation from phylogenetically older to younger forms, 

 and therefore, analogous to the organ of a growing individual, 

 pass through ancestral stages, as, e. g.> do the septa of a cephalo- 

 pod shell, demonstrates how closely the zooids of this colony 

 were united into one organism, and that practically they were 



